diff --git a/README.md b/README.md
index 861788514..f6d79c56d 100644
--- a/README.md
+++ b/README.md
@@ -25,6 +25,7 @@
 - [Categorical DQN (C51)](https://arxiv.org/pdf/1707.06887.pdf)
 - [Quantile Regression DQN (QRDQN)](https://arxiv.org/pdf/1710.10044.pdf)
 - [Policy Gradient (PG)](https://papers.nips.cc/paper/1713-policy-gradient-methods-for-reinforcement-learning-with-function-approximation.pdf)
+- [Natural Policy Gradient (NPG)](https://proceedings.neurips.cc/paper/2001/file/4b86abe48d358ecf194c56c69108433e-Paper.pdf)
 - [Advantage Actor-Critic (A2C)](https://openai.com/blog/baselines-acktr-a2c/)
 - [Trust Region Policy Optimization (TRPO)](https://arxiv.org/pdf/1502.05477.pdf)
 - [Proximal Policy Optimization (PPO)](https://arxiv.org/pdf/1707.06347.pdf)
diff --git a/docs/api/tianshou.policy.rst b/docs/api/tianshou.policy.rst
index 0664e0107..aa24897c4 100644
--- a/docs/api/tianshou.policy.rst
+++ b/docs/api/tianshou.policy.rst
@@ -43,6 +43,11 @@ On-policy
    :undoc-members:
    :show-inheritance:
 
+.. autoclass:: tianshou.policy.NPGPolicy
+   :members:
+   :undoc-members:
+   :show-inheritance:
+
 .. autoclass:: tianshou.policy.A2CPolicy
    :members:
    :undoc-members:
diff --git a/docs/index.rst b/docs/index.rst
index bd19d5618..08ed3245d 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -15,6 +15,7 @@ Welcome to Tianshou!
 * :class:`~tianshou.policy.C51Policy` `Categorical DQN <https://arxiv.org/pdf/1707.06887.pdf>`_
 * :class:`~tianshou.policy.QRDQNPolicy` `Quantile Regression DQN <https://arxiv.org/pdf/1710.10044.pdf>`_
 * :class:`~tianshou.policy.PGPolicy` `Policy Gradient <https://papers.nips.cc/paper/1713-policy-gradient-methods-for-reinforcement-learning-with-function-approximation.pdf>`_
+* :class:`~tianshou.policy.NPGPolicy` `Natural Policy Gradient <https://proceedings.neurips.cc/paper/2001/file/4b86abe48d358ecf194c56c69108433e-Paper.pdf>`_
 * :class:`~tianshou.policy.A2CPolicy` `Advantage Actor-Critic <https://openai.com/blog/baselines-acktr-a2c/>`_
 * :class:`~tianshou.policy.TRPOPolicy` `Trust Region Policy Optimization <https://arxiv.org/pdf/1502.05477.pdf>`_
 * :class:`~tianshou.policy.PPOPolicy` `Proximal Policy Optimization <https://arxiv.org/pdf/1707.06347.pdf>`_
diff --git a/test/continuous/test_npg.py b/test/continuous/test_npg.py
new file mode 100644
index 000000000..d5172fa8b
--- /dev/null
+++ b/test/continuous/test_npg.py
@@ -0,0 +1,136 @@
+import os
+import gym
+import torch
+import pprint
+import argparse
+import numpy as np
+from torch import nn
+from torch.utils.tensorboard import SummaryWriter
+from torch.distributions import Independent, Normal
+
+from tianshou.policy import NPGPolicy
+from tianshou.utils import BasicLogger
+from tianshou.env import DummyVectorEnv
+from tianshou.utils.net.common import Net
+from tianshou.trainer import onpolicy_trainer
+from tianshou.data import Collector, VectorReplayBuffer
+from tianshou.utils.net.continuous import ActorProb, Critic
+
+
+def get_args():
+    parser = argparse.ArgumentParser()
+    parser.add_argument('--task', type=str, default='Pendulum-v0')
+    parser.add_argument('--seed', type=int, default=1)
+    parser.add_argument('--buffer-size', type=int, default=50000)
+    parser.add_argument('--lr', type=float, default=1e-3)
+    parser.add_argument('--gamma', type=float, default=0.95)
+    parser.add_argument('--epoch', type=int, default=5)
+    parser.add_argument('--step-per-epoch', type=int, default=50000)
+    parser.add_argument('--step-per-collect', type=int, default=2048)
+    parser.add_argument('--repeat-per-collect', type=int,
+                        default=2)  # theoretically it should be 1
+    parser.add_argument('--batch-size', type=int, default=99999)
+    parser.add_argument('--hidden-sizes', type=int, nargs='*', default=[64, 64])
+    parser.add_argument('--training-num', type=int, default=16)
+    parser.add_argument('--test-num', type=int, default=10)
+    parser.add_argument('--logdir', type=str, default='log')
+    parser.add_argument('--render', type=float, default=0.)
+    parser.add_argument(
+        '--device', type=str,
+        default='cuda' if torch.cuda.is_available() else 'cpu')
+    # npg special
+    parser.add_argument('--gae-lambda', type=float, default=0.95)
+    parser.add_argument('--rew-norm', type=int, default=1)
+    parser.add_argument('--norm-adv', type=int, default=1)
+    parser.add_argument('--optim-critic-iters', type=int, default=5)
+    parser.add_argument('--actor-step-size', type=float, default=0.5)
+    args = parser.parse_known_args()[0]
+    return args
+
+
+def test_npg(args=get_args()):
+    env = gym.make(args.task)
+    if args.task == 'Pendulum-v0':
+        env.spec.reward_threshold = -250
+    args.state_shape = env.observation_space.shape or env.observation_space.n
+    args.action_shape = env.action_space.shape or env.action_space.n
+    args.max_action = env.action_space.high[0]
+    # you can also use tianshou.env.SubprocVectorEnv
+    # train_envs = gym.make(args.task)
+    train_envs = DummyVectorEnv(
+        [lambda: gym.make(args.task) for _ in range(args.training_num)])
+    # test_envs = gym.make(args.task)
+    test_envs = DummyVectorEnv(
+        [lambda: gym.make(args.task) for _ in range(args.test_num)])
+    # seed
+    np.random.seed(args.seed)
+    torch.manual_seed(args.seed)
+    train_envs.seed(args.seed)
+    test_envs.seed(args.seed)
+    # model
+    net = Net(args.state_shape, hidden_sizes=args.hidden_sizes,
+              activation=nn.Tanh, device=args.device)
+    actor = ActorProb(net, args.action_shape, max_action=args.max_action,
+                      unbounded=True, device=args.device).to(args.device)
+    critic = Critic(Net(
+        args.state_shape, hidden_sizes=args.hidden_sizes, device=args.device,
+        activation=nn.Tanh), device=args.device).to(args.device)
+    # orthogonal initialization
+    for m in list(actor.modules()) + list(critic.modules()):
+        if isinstance(m, torch.nn.Linear):
+            torch.nn.init.orthogonal_(m.weight)
+            torch.nn.init.zeros_(m.bias)
+    optim = torch.optim.Adam(set(
+        actor.parameters()).union(critic.parameters()), lr=args.lr)
+
+    # replace DiagGuassian with Independent(Normal) which is equivalent
+    # pass *logits to be consistent with policy.forward
+    def dist(*logits):
+        return Independent(Normal(*logits), 1)
+
+    policy = NPGPolicy(
+        actor, critic, optim, dist,
+        discount_factor=args.gamma,
+        reward_normalization=args.rew_norm,
+        advantage_normalization=args.norm_adv,
+        gae_lambda=args.gae_lambda,
+        action_space=env.action_space,
+        optim_critic_iters=args.optim_critic_iters,
+        actor_step_size=args.actor_step_size)
+    # collector
+    train_collector = Collector(
+        policy, train_envs,
+        VectorReplayBuffer(args.buffer_size, len(train_envs)))
+    test_collector = Collector(policy, test_envs)
+    # log
+    log_path = os.path.join(args.logdir, args.task, 'npg')
+    writer = SummaryWriter(log_path)
+    logger = BasicLogger(writer)
+
+    def save_fn(policy):
+        torch.save(policy.state_dict(), os.path.join(log_path, 'policy.pth'))
+
+    def stop_fn(mean_rewards):
+        return mean_rewards >= env.spec.reward_threshold
+
+    # trainer
+    result = onpolicy_trainer(
+        policy, train_collector, test_collector, args.epoch,
+        args.step_per_epoch, args.repeat_per_collect, args.test_num, args.batch_size,
+        step_per_collect=args.step_per_collect, stop_fn=stop_fn, save_fn=save_fn,
+        logger=logger)
+    assert stop_fn(result['best_reward'])
+
+    if __name__ == '__main__':
+        pprint.pprint(result)
+        # Let's watch its performance!
+        env = gym.make(args.task)
+        policy.eval()
+        collector = Collector(policy, env)
+        result = collector.collect(n_episode=1, render=args.render)
+        rews, lens = result["rews"], result["lens"]
+        print(f"Final reward: {rews.mean()}, length: {lens.mean()}")
+
+
+if __name__ == '__main__':
+    test_npg()
diff --git a/test/continuous/test_trpo.py b/test/continuous/test_trpo.py
index 4b2dc08dc..9db4f449c 100644
--- a/test/continuous/test_trpo.py
+++ b/test/continuous/test_trpo.py
@@ -27,8 +27,7 @@ def get_args():
     parser.add_argument('--epoch', type=int, default=5)
     parser.add_argument('--step-per-epoch', type=int, default=50000)
     parser.add_argument('--step-per-collect', type=int, default=2048)
-    parser.add_argument('--repeat-per-collect', type=int,
-                        default=2)  # theoretically it should be 1
+    parser.add_argument('--repeat-per-collect', type=int, default=1)
     parser.add_argument('--batch-size', type=int, default=99999)
     parser.add_argument('--hidden-sizes', type=int, nargs='*', default=[64, 64])
     parser.add_argument('--training-num', type=int, default=16)
@@ -43,7 +42,7 @@ def get_args():
     parser.add_argument('--rew-norm', type=int, default=1)
     parser.add_argument('--norm-adv', type=int, default=1)
     parser.add_argument('--optim-critic-iters', type=int, default=5)
-    parser.add_argument('--max-kl', type=float, default=0.01)
+    parser.add_argument('--max-kl', type=float, default=0.005)
     parser.add_argument('--backtrack-coeff', type=float, default=0.8)
     parser.add_argument('--max-backtracks', type=int, default=10)
 
diff --git a/tianshou/policy/__init__.py b/tianshou/policy/__init__.py
index d087a89b1..f0177d9cb 100644
--- a/tianshou/policy/__init__.py
+++ b/tianshou/policy/__init__.py
@@ -5,6 +5,7 @@
 from tianshou.policy.modelfree.qrdqn import QRDQNPolicy
 from tianshou.policy.modelfree.pg import PGPolicy
 from tianshou.policy.modelfree.a2c import A2CPolicy
+from tianshou.policy.modelfree.npg import NPGPolicy
 from tianshou.policy.modelfree.ddpg import DDPGPolicy
 from tianshou.policy.modelfree.ppo import PPOPolicy
 from tianshou.policy.modelfree.trpo import TRPOPolicy
@@ -25,6 +26,7 @@
     "QRDQNPolicy",
     "PGPolicy",
     "A2CPolicy",
+    "NPGPolicy",
     "DDPGPolicy",
     "PPOPolicy",
     "TRPOPolicy",
diff --git a/tianshou/policy/modelfree/npg.py b/tianshou/policy/modelfree/npg.py
new file mode 100644
index 000000000..abb6396af
--- /dev/null
+++ b/tianshou/policy/modelfree/npg.py
@@ -0,0 +1,182 @@
+import torch
+import numpy as np
+from torch import nn
+import torch.nn.functional as F
+from typing import Any, Dict, List, Type
+from torch.distributions import kl_divergence
+
+
+from tianshou.policy import A2CPolicy
+from tianshou.data import Batch, ReplayBuffer
+
+
+class NPGPolicy(A2CPolicy):
+    """Implementation of Natural Policy Gradient.
+
+    https://proceedings.neurips.cc/paper/2001/file/4b86abe48d358ecf194c56c69108433e-Paper.pdf
+
+    :param torch.nn.Module actor: the actor network following the rules in
+        :class:`~tianshou.policy.BasePolicy`. (s -> logits)
+    :param torch.nn.Module critic: the critic network. (s -> V(s))
+    :param torch.optim.Optimizer optim: the optimizer for actor and critic network.
+    :param dist_fn: distribution class for computing the action.
+    :type dist_fn: Type[torch.distributions.Distribution]
+    :param bool advantage_normalization: whether to do per mini-batch advantage
+        normalization. Default to True.
+    :param int optim_critic_iters: Number of times to optimize critic network per
+        update. Default to 5.
+    :param float gae_lambda: in [0, 1], param for Generalized Advantage Estimation.
+        Default to 0.95.
+    :param bool reward_normalization: normalize estimated values to have std close to
+        1. Default to False.
+    :param int max_batchsize: the maximum size of the batch when computing GAE,
+        depends on the size of available memory and the memory cost of the
+        model; should be as large as possible within the memory constraint.
+        Default to 256.
+    :param bool action_scaling: whether to map actions from range [-1, 1] to range
+        [action_spaces.low, action_spaces.high]. Default to True.
+    :param str action_bound_method: method to bound action to range [-1, 1], can be
+        either "clip" (for simply clipping the action), "tanh" (for applying tanh
+        squashing) for now, or empty string for no bounding. Default to "clip".
+    :param Optional[gym.Space] action_space: env's action space, mandatory if you want
+        to use option "action_scaling" or "action_bound_method". Default to None.
+    :param lr_scheduler: a learning rate scheduler that adjusts the learning rate in
+        optimizer in each policy.update(). Default to None (no lr_scheduler).
+    """
+
+    def __init__(
+        self,
+        actor: torch.nn.Module,
+        critic: torch.nn.Module,
+        optim: torch.optim.Optimizer,
+        dist_fn: Type[torch.distributions.Distribution],
+        advantage_normalization: bool = True,
+        optim_critic_iters: int = 5,
+        actor_step_size: float = 0.5,
+        **kwargs: Any,
+    ) -> None:
+        super().__init__(actor, critic, optim, dist_fn, **kwargs)
+        del self._weight_vf, self._weight_ent, self._grad_norm
+        self._norm_adv = advantage_normalization
+        self._optim_critic_iters = optim_critic_iters
+        self._step_size = actor_step_size
+        # adjusts Hessian-vector product calculation for numerical stability
+        self._damping = 0.1
+
+    def process_fn(
+        self, batch: Batch, buffer: ReplayBuffer, indice: np.ndarray
+    ) -> Batch:
+        batch = super().process_fn(batch, buffer, indice)
+        old_log_prob = []
+        with torch.no_grad():
+            for b in batch.split(self._batch, shuffle=False, merge_last=True):
+                old_log_prob.append(self(b).dist.log_prob(b.act))
+        batch.logp_old = torch.cat(old_log_prob, dim=0)
+        if self._norm_adv:
+            batch.adv = (batch.adv - batch.adv.mean()) / batch.adv.std()
+        return batch
+
+    def learn(  # type: ignore
+        self, batch: Batch, batch_size: int, repeat: int, **kwargs: Any
+    ) -> Dict[str, List[float]]:
+        actor_losses, vf_losses, kls = [], [], []
+        for step in range(repeat):
+            for b in batch.split(batch_size, merge_last=True):
+                # optimize actor
+                # direction: calculate villia gradient
+                dist = self(b).dist  # TODO could come from batch
+                ratio = (dist.log_prob(b.act) - b.logp_old).exp().float()
+                ratio = ratio.reshape(ratio.size(0), -1).transpose(0, 1)
+                actor_loss = -(ratio * b.adv).mean()
+                flat_grads = self._get_flat_grad(
+                    actor_loss, self.actor, retain_graph=True).detach()
+
+                # direction: calculate natural gradient
+                with torch.no_grad():
+                    old_dist = self(b).dist
+
+                kl = kl_divergence(old_dist, dist).mean()
+                # calculate first order gradient of kl with respect to theta
+                flat_kl_grad = self._get_flat_grad(kl, self.actor, create_graph=True)
+                search_direction = -self._conjugate_gradients(
+                    flat_grads, flat_kl_grad, nsteps=10)
+
+                # step
+                with torch.no_grad():
+                    flat_params = torch.cat([param.data.view(-1)
+                                             for param in self.actor.parameters()])
+                    new_flat_params = flat_params + self._step_size * search_direction
+                    self._set_from_flat_params(self.actor, new_flat_params)
+                    new_dist = self(b).dist
+                    kl = kl_divergence(old_dist, new_dist).mean()
+
+                # optimize citirc
+                for _ in range(self._optim_critic_iters):
+                    value = self.critic(b.obs).flatten()
+                    vf_loss = F.mse_loss(b.returns, value)
+                    self.optim.zero_grad()
+                    vf_loss.backward()
+                    self.optim.step()
+
+                actor_losses.append(actor_loss.item())
+                vf_losses.append(vf_loss.item())
+                kls.append(kl.item())
+
+        # update learning rate if lr_scheduler is given
+        if self.lr_scheduler is not None:
+            self.lr_scheduler.step()
+
+        return {
+            "loss/actor": actor_losses,
+            "loss/vf": vf_losses,
+            "kl": kls,
+        }
+
+    def _MVP(self, v: torch.Tensor, flat_kl_grad: torch.Tensor) -> torch.Tensor:
+        """Matrix vector product."""
+        # caculate second order gradient of kl with respect to theta
+        kl_v = (flat_kl_grad * v).sum()
+        flat_kl_grad_grad = self._get_flat_grad(
+            kl_v, self.actor, retain_graph=True).detach()
+        return flat_kl_grad_grad + v * self._damping
+
+    def _conjugate_gradients(
+        self,
+        b: torch.Tensor,
+        flat_kl_grad: torch.Tensor,
+        nsteps: int = 10,
+        residual_tol: float = 1e-10
+    ) -> torch.Tensor:
+        x = torch.zeros_like(b)
+        r, p = b.clone(), b.clone()
+        # Note: should be 'r, p = b - MVP(x)', but for x=0, MVP(x)=0.
+        # Change if doing warm start.
+        rdotr = r.dot(r)
+        for i in range(nsteps):
+            z = self._MVP(p, flat_kl_grad)
+            alpha = rdotr / p.dot(z)
+            x += alpha * p
+            r -= alpha * z
+            new_rdotr = r.dot(r)
+            if new_rdotr < residual_tol:
+                break
+            p = r + new_rdotr / rdotr * p
+            rdotr = new_rdotr
+        return x
+
+    def _get_flat_grad(
+        self, y: torch.Tensor, model: nn.Module, **kwargs: Any
+    ) -> torch.Tensor:
+        grads = torch.autograd.grad(y, model.parameters(), **kwargs)  # type: ignore
+        return torch.cat([grad.reshape(-1) for grad in grads])
+
+    def _set_from_flat_params(
+        self, model: nn.Module, flat_params: torch.Tensor
+    ) -> nn.Module:
+        prev_ind = 0
+        for param in model.parameters():
+            flat_size = int(np.prod(list(param.size())))
+            param.data.copy_(
+                flat_params[prev_ind:prev_ind + flat_size].view(param.size()))
+            prev_ind += flat_size
+        return model
diff --git a/tianshou/policy/modelfree/trpo.py b/tianshou/policy/modelfree/trpo.py
index 32ba13976..9d456878c 100644
--- a/tianshou/policy/modelfree/trpo.py
+++ b/tianshou/policy/modelfree/trpo.py
@@ -1,56 +1,15 @@
 import torch
 import warnings
-import numpy as np
-from torch import nn
 import torch.nn.functional as F
+from typing import Any, Dict, List, Type
 from torch.distributions import kl_divergence
-from typing import Any, Dict, List, Type, Callable
-
-
-from tianshou.policy import A2CPolicy
-from tianshou.data import Batch, ReplayBuffer
-
-
-def _conjugate_gradients(
-    Avp: Callable[[torch.Tensor], torch.Tensor],
-    b: torch.Tensor,
-    nsteps: int = 10,
-    residual_tol: float = 1e-10
-) -> torch.Tensor:
-    x = torch.zeros_like(b)
-    r, p = b.clone(), b.clone()
-    # Note: should be 'r, p = b - A(x)', but for x=0, A(x)=0.
-    # Change if doing warm start.
-    rdotr = r.dot(r)
-    for i in range(nsteps):
-        z = Avp(p)
-        alpha = rdotr / p.dot(z)
-        x += alpha * p
-        r -= alpha * z
-        new_rdotr = r.dot(r)
-        if new_rdotr < residual_tol:
-            break
-        p = r + new_rdotr / rdotr * p
-        rdotr = new_rdotr
-    return x
-
-
-def _get_flat_grad(y: torch.Tensor, model: nn.Module, **kwargs: Any) -> torch.Tensor:
-    grads = torch.autograd.grad(y, model.parameters(), **kwargs)  # type: ignore
-    return torch.cat([grad.reshape(-1) for grad in grads])
-
-
-def _set_from_flat_params(model: nn.Module, flat_params: torch.Tensor) -> nn.Module:
-    prev_ind = 0
-    for param in model.parameters():
-        flat_size = int(np.prod(list(param.size())))
-        param.data.copy_(
-            flat_params[prev_ind:prev_ind + flat_size].view(param.size()))
-        prev_ind += flat_size
-    return model
-
-
-class TRPOPolicy(A2CPolicy):
+
+
+from tianshou.data import Batch
+from tianshou.policy import NPGPolicy
+
+
+class TRPOPolicy(NPGPolicy):
     """Implementation of Trust Region Policy Optimization. arXiv:1502.05477.
 
     :param torch.nn.Module actor: the actor network following the rules in
@@ -94,35 +53,16 @@ def __init__(
         critic: torch.nn.Module,
         optim: torch.optim.Optimizer,
         dist_fn: Type[torch.distributions.Distribution],
-        advantage_normalization: bool = True,
-        optim_critic_iters: int = 5,
         max_kl: float = 0.01,
         backtrack_coeff: float = 0.8,
         max_backtracks: int = 10,
         **kwargs: Any,
     ) -> None:
         super().__init__(actor, critic, optim, dist_fn, **kwargs)
-        del self._weight_vf, self._weight_ent, self._grad_norm
-        self._norm_adv = advantage_normalization
-        self._optim_critic_iters = optim_critic_iters
+        del self._step_size
         self._max_backtracks = max_backtracks
         self._delta = max_kl
         self._backtrack_coeff = backtrack_coeff
-        # adjusts Hessian-vector product calculation for numerical stability
-        self.__damping = 0.1
-
-    def process_fn(
-        self, batch: Batch, buffer: ReplayBuffer, indice: np.ndarray
-    ) -> Batch:
-        batch = super().process_fn(batch, buffer, indice)
-        old_log_prob = []
-        with torch.no_grad():
-            for b in batch.split(self._batch, shuffle=False, merge_last=True):
-                old_log_prob.append(self(b).dist.log_prob(b.act))
-        batch.logp_old = torch.cat(old_log_prob, dim=0)
-        if self._norm_adv:
-            batch.adv = (batch.adv - batch.adv.mean()) / batch.adv.std()
-        return batch
 
     def learn(  # type: ignore
         self, batch: Batch, batch_size: int, repeat: int, **kwargs: Any
@@ -136,7 +76,7 @@ def learn(  # type: ignore
                 ratio = (dist.log_prob(b.act) - b.logp_old).exp().float()
                 ratio = ratio.reshape(ratio.size(0), -1).transpose(0, 1)
                 actor_loss = -(ratio * b.adv).mean()
-                flat_grads = _get_flat_grad(
+                flat_grads = self._get_flat_grad(
                     actor_loss, self.actor, retain_graph=True).detach()
 
                 # direction: calculate natural gradient
@@ -145,20 +85,14 @@ def learn(  # type: ignore
 
                 kl = kl_divergence(old_dist, dist).mean()
                 # calculate first order gradient of kl with respect to theta
-                flat_kl_grad = _get_flat_grad(kl, self.actor, create_graph=True)
-
-                def MVP(v: torch.Tensor) -> torch.Tensor:  # matrix vector product
-                    # caculate second order gradient of kl with respect to theta
-                    kl_v = (flat_kl_grad * v).sum()
-                    flat_kl_grad_grad = _get_flat_grad(
-                        kl_v, self.actor, retain_graph=True).detach()
-                    return flat_kl_grad_grad + v * self.__damping
-
-                search_direction = -_conjugate_gradients(MVP, flat_grads, nsteps=10)
+                flat_kl_grad = self._get_flat_grad(kl, self.actor, create_graph=True)
+                search_direction = -self._conjugate_gradients(
+                    flat_grads, flat_kl_grad, nsteps=10)
 
                 # stepsize: calculate max stepsize constrained by kl bound
                 step_size = torch.sqrt(2 * self._delta / (
-                    search_direction * MVP(search_direction)).sum(0, keepdim=True))
+                    search_direction * self._MVP(search_direction, flat_kl_grad)
+                ).sum(0, keepdim=True))
 
                 # stepsize: linesearch stepsize
                 with torch.no_grad():
@@ -166,7 +100,7 @@ def MVP(v: torch.Tensor) -> torch.Tensor:  # matrix vector product
                                              for param in self.actor.parameters()])
                     for i in range(self._max_backtracks):
                         new_flat_params = flat_params + step_size * search_direction
-                        _set_from_flat_params(self.actor, new_flat_params)
+                        self._set_from_flat_params(self.actor, new_flat_params)
                         # calculate kl and if in bound, loss actually down
                         new_dist = self(b).dist
                         new_dratio = (
@@ -183,7 +117,7 @@ def MVP(v: torch.Tensor) -> torch.Tensor:  # matrix vector product
                         elif i < self._max_backtracks - 1:
                             step_size = step_size * self._backtrack_coeff
                         else:
-                            _set_from_flat_params(self.actor, new_flat_params)
+                            self._set_from_flat_params(self.actor, new_flat_params)
                             step_size = torch.tensor([0.0])
                             warnings.warn("Line search failed! It seems hyperparamters"
                                           " are poor and need to be changed.")