diff --git a/Cargo.lock b/Cargo.lock
index e9174a6..326d429 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -12,6 +12,10 @@ version = "0.1.0"
 name = "convert_sorted_list_to_binary_search_tree"
 version = "0.1.0"
 
+[[package]]
+name = "count_primes"
+version = "0.1.0"
+
 [[package]]
 name = "course_schedule_iii"
 version = "0.1.0"
diff --git a/Cargo.toml b/Cargo.toml
index 298d8ba..ce301d0 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -20,4 +20,5 @@ members = [
     "convert_sorted_list_to_binary_search_tree",
     "delete_operation_for_two_strings",
     "construct_target_array_with_multiple_sums",
+    "count_primes",
 ]
diff --git a/count_primes/Cargo.toml b/count_primes/Cargo.toml
new file mode 100644
index 0000000..220c254
--- /dev/null
+++ b/count_primes/Cargo.toml
@@ -0,0 +1,9 @@
+[package]
+name = "count_primes"
+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/count_primes/src/lib.rs b/count_primes/src/lib.rs
new file mode 100644
index 0000000..10af1b9
--- /dev/null
+++ b/count_primes/src/lib.rs
@@ -0,0 +1,62 @@
+/*!
+ * https://leetcode.com/explore/challenge/card/may-leetcoding-challenge-2021/599/week-2-may-8th-may-14th/3738/
+ */
+
+pub struct Solution {}
+
+impl Solution {
+    // /// a normal way
+    // pub fn count_primes(n: i32) -> i32 {
+    //     (0..n)
+    //         .filter(|x| *x == 2 || *x % 2 != 0)
+    //         .filter(|x| Self::is_prime(x))
+    //         .count() as i32
+    // }
+
+    // fn is_prime(x: &i32) -> bool {
+    //     *x > 1 && !(2..*x).filter(|i| i * i <= *x).any(|i| *x % i == 0)
+    // }
+
+    pub fn count_primes(n: i32) -> i32 {
+        if n <= 2 {
+            return 0;
+        }
+
+        let n = n as usize;
+        let mut nums = vec![true; n];
+        nums[0] = false;
+        nums[1] = false;
+        let mut i = 2;
+        while i * i < n {
+            if nums[i] {
+                let mut j = 2;
+                while i * j < n {
+                    nums[i * j] = false;
+                    j += 1;
+                }
+            }
+            i += 1;
+        }
+        nums.iter().filter(|&&x| x).count() as i32
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn example_1() {
+        assert_eq!(Solution::count_primes(10), 4);
+    }
+
+    #[test]
+    fn example_2() {
+        assert_eq!(Solution::count_primes(0), 0);
+    }
+
+    #[test]
+    fn example_3() {
+        assert_eq!(Solution::count_primes(1), 0);
+    }
+}