diff --git a/tianshou/core/mcts/evaluator.py b/tianshou/core/mcts/evaluator.py
index 85041be..aacdbe4 100644
--- a/tianshou/core/mcts/evaluator.py
+++ b/tianshou/core/mcts/evaluator.py
@@ -1,5 +1,6 @@
 import numpy as np
 
+
 class evaluator(object):
     def __init__(self, env, action_num):
         self.env = env
@@ -8,6 +9,7 @@ class evaluator(object):
     def __call__(self, state):
         raise NotImplementedError("Need to implement the evaluator")
 
+
 class rollout_policy(evaluator):
     def __init__(self, env, action_num):
         super(rollout_policy, self).__init__(env, action_num)
@@ -15,6 +17,11 @@ class rollout_policy(evaluator):
 
     def __call__(self, state):
         # TODO: prior for rollout policy
-        while not self.is_terminated:
-            action = np.random.randint(0,self.action_num)
-            state, is_terminated = self.env.step_forward(state, action)
\ No newline at end of file
+        total_reward = 0
+        action = np.random.randint(0, self.action_num)
+        state, reward = self.env.step_forward(state, action)
+        while state is not None:
+            action = np.random.randint(0, self.action_num)
+            state, reward = self.env.step_forward(state, action)
+            total_reward += reward
+        return reward
diff --git a/tianshou/core/mcts/mcts.py b/tianshou/core/mcts/mcts.py
index 521b455..1bdc0ff 100644
--- a/tianshou/core/mcts/mcts.py
+++ b/tianshou/core/mcts/mcts.py
@@ -2,7 +2,7 @@ import numpy as np
 import math
 import time
 
-c_puct = 1
+c_puct = 5
 
 
 class MCTSNode(object):
@@ -14,15 +14,12 @@ class MCTSNode(object):
         self.action_num = action_num
         self.prior = prior
 
-    def selection(self):
+    def selection(self, simulator):
         raise NotImplementedError("Need to implement function selection")
 
-    def backpropagation(self, action, value):
+    def backpropagation(self, action):
         raise NotImplementedError("Need to implement function backpropagation")
 
-    def expansion(self, simulator, action):
-        raise NotImplementedError("Need to implement function expansion")
-
     def simulation(self, state, evaluator):
         raise NotImplementedError("Need to implement function simulation")
 
@@ -34,40 +31,24 @@ class UCTNode(MCTSNode):
         self.W = np.zeros([action_num])
         self.N = np.zeros([action_num])
         self.ucb = self.Q + c_puct * self.prior * math.sqrt(np.sum(self.N)) / (self.N + 1)
-        self.is_terminated = False
 
-    def selection(self):
-        if not self.is_terminated:
-            action = np.argmax(self.ucb)
-            if action in self.children.keys():
-                node, action = self.children[action].selection()
-            else:
-                node = self
+    def selection(self, simulator):
+        action = np.argmax(self.ucb)
+        if action in self.children.keys():
+            return self.children[action].selection(simulator)
         else:
-            action = None
-            node = self
-        return node, action
+            self.children[action] = ActionNode(self, action)
+            return self.children[action].selection(simulator)
 
-    def backpropagation(self, action, value):
-        if action is None:
-            if self.parent is not None:
-                self.parent.backpropagation(self.action, value)
-        else:
-            self.N[action] += 1
-            self.W[action] += value
-            for i in range(self.action_num):
-                if self.N[i] != 0:
-                    self.Q[i] = (self.W[i] + 0.)/self.N[i]
-            self.ucb = self.Q + c_puct * self.prior * math.sqrt(np.sum(self.N)) / (self.N + 1.)
-            if self.parent is not None:
-                self.parent.backpropagation(self.action, value)
-
-    def expansion(self, simulator, action):
-        next_state, is_terminated = simulator.step_forward(self.state, action)
-        # TODO: Let users/evaluator give the prior
-        prior = np.ones([self.action_num]) / self.action_num
-        self.children[action] = UCTNode(self, action, next_state, self.action_num, prior)
-        self.children[action].is_terminated = is_terminated
+    def backpropagation(self, action):
+        self.N[action] += 1
+        self.W[action] += self.children[action].reward
+        for i in range(self.action_num):
+            if self.N[i] != 0:
+                self.Q[i] = (self.W[i] + 0.) / self.N[i]
+        self.ucb = self.Q + c_puct * self.prior * math.sqrt(np.sum(self.N)) / (self.N + 1.)
+        if self.parent is not None:
+            self.parent.backpropagation(self.children[action].reward)
 
     def simulation(self, evaluator, state):
         value = evaluator(state)
@@ -90,13 +71,38 @@ class ActionNode:
         self.parent = parent
         self.action = action
         self.children = {}
-        self.value = {}
+        self.next_state = None
+        self.reward = 0
+
+    def selection(self, simulator):
+        self.next_state, self.reward = simulator.step_forward(self.parent.state, self.action)
+        if self.next_state is not None:
+            if self.next_state in self.children.keys():
+                return self.children[self.next_state].selection(simulator)
+            else:
+                return self.parent, self.action
+        else:
+            return self.parent, self.action
+
+    def expansion(self, action_num):
+        # TODO: Let users/evaluator give the prior
+        if self.next_state is not None:
+            prior = np.ones([action_num]) / action_num
+            self.children[self.next_state] = UCTNode(self.parent, self.action, self.next_state, action_num, prior)
+            return True
+        else:
+            return False
+
+    def backpropagation(self, value):
+        self.reward += value
+        self.parent.backpropagation(self.action)
 
 
 class MCTS:
     def __init__(self, simulator, evaluator, root, action_num, prior, method="UCT", max_step=None, max_time=None):
         self.simulator = simulator
         self.evaluator = evaluator
+        self.action_num = action_num
         if method == "UCT":
             self.root = UCTNode(None, None, root, action_num, prior)
         if method == "TS":
@@ -111,21 +117,25 @@ class MCTS:
             raise ValueError("Need a stop criteria!")
         while (max_step is not None and self.step < self.max_step or max_step is None) \
                 and (max_time is not None and time.time() - self.start_time < self.max_time or max_time is None):
-            print(self.root.Q)
+            print("Q={}".format(self.root.Q))
+            print("N={}".format(self.root.N))
+            print("W={}".format(self.root.W))
+            print("UCB={}".format(self.root.ucb))
+            print("\n")
             self.expand()
             if max_step is not None:
                 self.step += 1
 
     def expand(self):
-        node, new_action = self.root.selection()
-        if new_action is None:
-            value = node.simulation(self.evaluator, node.state)
-            node.backpropagation(new_action, value)
+        node, new_action = self.root.selection(self.simulator)
+        success = node.children[new_action].expansion(self.action_num)
+        if success:
+            value = node.simulation(self.evaluator, node.children[new_action].next_state)
+            node.children[new_action].backpropagation(value + 0.)
         else:
-            node.expansion(self.simulator, new_action)
-            value = node.simulation(self.evaluator, node.children[new_action].state)
-            node.backpropagation(new_action, value)
+            value = node.simulation(self.evaluator, node.state)
+            node.parent.children[node.action].backpropagation(value + 0.)
 
 
-if __name__=="__main__":
-    pass
\ No newline at end of file
+if __name__ == "__main__":
+    pass
diff --git a/tianshou/core/mcts/mcts_test.py b/tianshou/core/mcts/mcts_test.py
index f708b39..a0425b8 100644
--- a/tianshou/core/mcts/mcts_test.py
+++ b/tianshou/core/mcts/mcts_test.py
@@ -1,34 +1,39 @@
 import numpy as np
 from mcts import MCTS
-import matplotlib.pyplot as plt
+from evaluator import rollout_policy
+
 
 class TestEnv:
     def __init__(self, max_step=5):
         self.max_step = max_step
-        self.reward = {i:np.random.uniform() for i in range(2**max_step)}
+        self.reward = {i: np.random.uniform() for i in range(2 ** max_step)}
         # self.reward = {0:0.8, 1:0.2, 2:0.4, 3:0.6}
-        self.best = max(self.reward.items(), key=lambda x:x[1])
+        self.best = max(self.reward.items(), key=lambda x: x[1])
         # print("The best arm is {} with expected reward {}".format(self.best[0],self.best[1]))
         print(self.reward)
 
     def step_forward(self, state, action):
         if action != 0 and action != 1:
             raise ValueError("Action must be 0 or 1! Your action is {}".format(action))
-        if state[0] >= 2**state[1] or state[1] >= self.max_step:
+        if state[0] >= 2 ** state[1] or state[1] > self.max_step:
             raise ValueError("Invalid State! Your state is {}".format(state))
         # print("Operate action {} at state {}, timestep {}".format(action, state[0], state[1]))
-        new_state = [0,0]
-        new_state[0] = state[0] + 2**state[1]*action
-        new_state[1] = state[1] + 1
-        if new_state[1] == self.max_step:
-            reward = int(np.random.uniform() < self.reward[state[0]])
-            is_terminated = True
-        else:
+        if state[1] == self.max_step:
+            new_state = None
             reward = 0
-            is_terminated = False
-        return new_state, reward, is_terminated
+        else:
+            num = state[0] + 2 ** state[1] * action
+            step = state[1] + 1
+            new_state = (num, step)
+            if step == self.max_step:
+                reward = int(np.random.uniform() < self.reward[state[0]])
+            else:
+                reward = 0
+        return new_state, reward
 
-if __name__=="__main__":
-    env = TestEnv(3)
-    evaluator = lambda state: env.step_forward(state, action)
-    mcts = MCTS(env, evaluator, [0,0], 2, np.array([0.5,0.5]), max_step=1e4)
+
+if __name__ == "__main__":
+    env = TestEnv(1)
+    rollout = rollout_policy(env, 2)
+    evaluator = lambda state: rollout(state)
+    mcts = MCTS(env, evaluator, [0, 0], 2, np.array([0.5, 0.5]), max_step=1e4)