diff --git a/tianshou/core/mcts/evaluator.py b/tianshou/core/mcts/evaluator.py
new file mode 100644
index 0000000..85041be
--- /dev/null
+++ b/tianshou/core/mcts/evaluator.py
@@ -0,0 +1,20 @@
+import numpy as np
+
+class evaluator(object):
+    def __init__(self, env, action_num):
+        self.env = env
+        self.action_num = action_num
+
+    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)
+        self.is_terminated = False
+
+    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
diff --git a/tianshou/core/mcts/mcts.py b/tianshou/core/mcts/mcts.py
index 16b0ea0..521b455 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 = 5.
+c_puct = 1
 
 
 class MCTSNode(object):
@@ -17,7 +17,7 @@ class MCTSNode(object):
     def selection(self):
         raise NotImplementedError("Need to implement function selection")
 
-    def backpropagation(self, action, value, is_terminated):
+    def backpropagation(self, action, value):
         raise NotImplementedError("Need to implement function backpropagation")
 
     def expansion(self, simulator, action):
@@ -37,35 +37,41 @@ class UCTNode(MCTSNode):
         self.is_terminated = False
 
     def selection(self):
-        if self.is_terminated:
-            action = None
-        else:
+        if not self.is_terminated:
             action = np.argmax(self.ucb)
-        if action in self.children.keys():
-            self.children[action].selection()
+            if action in self.children.keys():
+                node, action = self.children[action].selection()
+            else:
+                node = self
         else:
-            return self, action
+            action = None
+            node = self
+        return node, action
 
-    def backpropagation(self, action, value, is_terminated):
-        self.is_terminated = is_terminated
-        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.parent.action, value)
+    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 = simulator.step_forward(self.state, 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 simulation(self, evaluator, state):
-        value, is_ternimated = evaluator(state)
-        return value, is_ternimated
+        value = evaluator(state)
+        return value
 
 
 class TSNode(MCTSNode):
@@ -105,24 +111,20 @@ 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)
             self.expand()
             if max_step is not None:
                 self.step += 1
 
     def expand(self):
-        print(self.root.Q)
-        print(self.root.N)
-        print(self.root.W)
         node, new_action = self.root.selection()
-        print(node.state, new_action)
         if new_action is None:
-            value, is_terminated = node.simulation(self.evaluator, node.state)
-            node.backpropagation(node.action, value, is_terminated)
-            print(value)
+            value = node.simulation(self.evaluator, node.state)
+            node.backpropagation(new_action, value)
         else:
             node.expansion(self.simulator, new_action)
-            value, is_terminated = node.simulation(self.evaluator, node.children[new_action].state)
-            node.backpropagation(new_action, value, is_terminated)
+            value = node.simulation(self.evaluator, node.children[new_action].state)
+            node.backpropagation(new_action, value)
 
 
 if __name__=="__main__":
diff --git a/tianshou/core/mcts/mcts_test.py b/tianshou/core/mcts/mcts_test.py
index 4c86736..f708b39 100644
--- a/tianshou/core/mcts/mcts_test.py
+++ b/tianshou/core/mcts/mcts_test.py
@@ -6,29 +6,29 @@ 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 = {0:0.8, 1:0.2, 2:0.4, 3:0.6}
         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("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!")
+            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:
-            raise ValueError("Invalid State!")
+            raise ValueError("Invalid State! Your state is {}".format(state))
         # print("Operate action {} at state {}, timestep {}".format(action, state[0], state[1]))
-        state[0] = state[0] + 2**state[1]*action
-        state[1] = state[1] + 1
-        return state
-
-    def evaluator(self, state):
-        if state[1] == self.max_step:
-            reward = int(np.random.uniform() > self.reward[state[0]])
+        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:
             reward = 0
             is_terminated = False
-        return reward, is_terminated
+        return new_state, reward, is_terminated
 
 if __name__=="__main__":
-    env = TestEnv(1)
-    evaluator = lambda state: env.evaluator(state)
-    mcts = MCTS(env, evaluator, [0,0], 2, np.ones([2])/2, max_step=1e4)
+    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)