diff --git a/Cargo.lock b/Cargo.lock
index 4c2f566..b112202 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1,5 +1,9 @@
 # This file is automatically @generated by Cargo.
 # It is not intended for manual editing.
+[[package]]
+name = "binary_tree_postorder_traversal"
+version = "0.1.0"
+
 [[package]]
 name = "kth_largest"
 version = "0.1.0"
diff --git a/Cargo.toml b/Cargo.toml
index 0aadfb1..a735f0a 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -4,4 +4,5 @@ members = [
     "kth_largest",
     "reverse_integer",
     "three_sum",
+    "binary_tree_postorder_traversal",
 ]
diff --git a/README.md b/README.md
index 32cbe3d..d717516 100644
--- a/README.md
+++ b/README.md
@@ -11,6 +11,12 @@ Resolving problems of LeetCode in RustLang.
 * [15. 3 Sum](./three_sum/src/lib.rs)
 * [215. Kth Largest Element in an Array](./kth_largest/src/lib.rs)
 
+## Document
+
+```shell
+cargo doc --open -p <package_name>
+```
+
 ## References
 
 * [LeetcodeTop](https://github.com/afatcoder/LeetcodeTop): Top Hit Leetcode problems in interviews.
diff --git a/binary_tree_postorder_traversal/Cargo.toml b/binary_tree_postorder_traversal/Cargo.toml
new file mode 100644
index 0000000..31326b8
--- /dev/null
+++ b/binary_tree_postorder_traversal/Cargo.toml
@@ -0,0 +1,9 @@
+[package]
+name = "binary_tree_postorder_traversal"
+version = "0.1.0"
+authors = ["Ryan Li <conbas2019@gmail.com>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
diff --git a/binary_tree_postorder_traversal/src/lib.rs b/binary_tree_postorder_traversal/src/lib.rs
new file mode 100644
index 0000000..496b8fb
--- /dev/null
+++ b/binary_tree_postorder_traversal/src/lib.rs
@@ -0,0 +1,81 @@
+/*!
+ * # 145. Binary Tree Postorder Traversal
+ *
+ * * [problem link](https://leetcode.com/problems/binary-tree-postorder-traversal/)
+ *
+ * ## What is Postorder Traversal
+ *
+ * We can find explanation in [Wiki](https://en.wikipedia.org/wiki/Tree_traversal#Post-order_(LRN)):
+ *
+ * > Post-order (LRN)
+ * >
+ * > 1. Traverse the left subtree by recursively calling the post-order function.
+ * > 2. Traverse the right subtree by recursively calling the post-order function.
+ * > 3. Access the data part of the current node.
+ */
+
+#![allow(dead_code)]
+
+#[derive(Debug, PartialEq, Eq)]
+pub struct TreeNode {
+    pub val: i32,
+    pub left: Option<Rc<RefCell<TreeNode>>>,
+    pub right: Option<Rc<RefCell<TreeNode>>>,
+}
+
+impl TreeNode {
+    #[inline]
+    pub fn new(val: i32) -> Self {
+        TreeNode {
+            val,
+            left: None,
+            right: None,
+        }
+    }
+}
+
+struct Solution {}
+
+// ----------------------------------------------------------------------------
+
+use std::cell::RefCell;
+use std::rc::Rc;
+
+impl Solution {
+    pub fn postorder_traversal(root: Option<Rc<RefCell<TreeNode>>>) -> Vec<i32> {
+        match root {
+            None => return vec![],
+            Some(n) => {
+                let left = n.borrow().left.clone();
+                let right = n.borrow().right.clone();
+                let val = n.borrow().val;
+                let left_vec = Self::postorder_traversal(left);
+                let right_vec = Self::postorder_traversal(right);
+                let val_vec = vec![val];
+                return [left_vec, right_vec, val_vec].concat();
+            }
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_example() {
+        let three = Some(Rc::new(RefCell::new(TreeNode::new(3))));
+        let two = Some(Rc::new(RefCell::new(TreeNode {
+            val: 2,
+            left: three,
+            right: None,
+        })));
+        let input = Some(Rc::new(RefCell::new(TreeNode {
+            val: 1,
+            left: None,
+            right: two,
+        })));
+        let output = vec![3, 2, 1];
+        assert_eq!(Solution::postorder_traversal(input), output);
+    }
+}