diff --git a/os/src/syscall/mod.rs b/os/src/syscall/mod.rs
index e5054e19..dbcf37cc 100644
--- a/os/src/syscall/mod.rs
+++ b/os/src/syscall/mod.rs
@@ -5,6 +5,7 @@ const SYSCALL_PIPE: usize = 59;
 const SYSCALL_READ: usize = 63;
 const SYSCALL_WRITE: usize = 64;
 const SYSCALL_EXIT: usize = 93;
+const SYSCALL_SLEEP: usize = 101;
 const SYSCALL_YIELD: usize = 124;
 const SYSCALL_GET_TIME: usize = 169;
 const SYSCALL_GETPID: usize = 172;
@@ -37,6 +38,7 @@ pub fn syscall(syscall_id: usize, args: [usize; 3]) -> isize {
         SYSCALL_READ => sys_read(args[0], args[1] as *const u8, args[2]),
         SYSCALL_WRITE => sys_write(args[0], args[1] as *const u8, args[2]),
         SYSCALL_EXIT => sys_exit(args[0] as i32),
+        SYSCALL_SLEEP => sys_sleep(args[0]),
         SYSCALL_YIELD => sys_yield(),
         SYSCALL_GET_TIME => sys_get_time(),
         SYSCALL_GETPID => sys_getpid(),
diff --git a/os/src/syscall/sync.rs b/os/src/syscall/sync.rs
index 1f080131..c4808c52 100644
--- a/os/src/syscall/sync.rs
+++ b/os/src/syscall/sync.rs
@@ -1,7 +1,16 @@
-use crate::task::current_process;
+use crate::task::{current_task, current_process, block_current_and_run_next};
 use crate::sync::{MutexSpin, MutexBlocking};
+use crate::timer::{get_time_ms, add_timer};
 use alloc::sync::Arc;
 
+pub fn sys_sleep(ms: usize) -> isize {
+    let expire_ms = get_time_ms() + ms;
+    let task = current_task().unwrap();
+    add_timer(expire_ms, task);
+    block_current_and_run_next();
+    0
+}
+
 pub fn sys_mutex_create(blocking: bool) -> isize {
     let process = current_process();
     let mut process_inner = process.inner_exclusive_access();
diff --git a/os/src/timer.rs b/os/src/timer.rs
index ad226e64..70c5d72b 100644
--- a/os/src/timer.rs
+++ b/os/src/timer.rs
@@ -1,6 +1,13 @@
+use core::cmp::Ordering;
+
 use riscv::register::time;
 use crate::sbi::set_timer;
 use crate::config::CLOCK_FREQ;
+use crate::task::{TaskControlBlock, add_task};
+use crate::sync::UPSafeCell;
+use alloc::collections::BinaryHeap;
+use alloc::sync::Arc;
+use lazy_static::*;
 
 const TICKS_PER_SEC: usize = 100;
 const MSEC_PER_SEC: usize = 1000;
@@ -16,3 +23,54 @@ pub fn get_time_ms() -> usize {
 pub fn set_next_trigger() {
     set_timer(get_time() + CLOCK_FREQ / TICKS_PER_SEC);
 }
+
+pub struct TimerCondVar {
+    pub expire_ms: usize,
+    pub task: Arc<TaskControlBlock>,
+}
+
+impl PartialEq for TimerCondVar {
+    fn eq(&self, other: &Self) -> bool {
+        self.expire_ms == other.expire_ms
+    }
+}
+impl Eq for TimerCondVar {}
+impl PartialOrd for TimerCondVar {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        let a = -(self.expire_ms as isize);
+        let b = -(other.expire_ms as isize);
+        Some(a.cmp(&b)) 
+    }
+}
+
+impl Ord for TimerCondVar {
+    fn cmp(&self, other: &Self) -> Ordering {
+        self.partial_cmp(other).unwrap() 
+    }
+}
+
+lazy_static! {
+    static ref TIMERS: UPSafeCell<BinaryHeap<TimerCondVar>> = unsafe { UPSafeCell::new(
+        BinaryHeap::<TimerCondVar>::new()    
+    )};
+}
+
+pub fn add_timer(expire_ms: usize, task: Arc<TaskControlBlock>) {
+    let mut timers = TIMERS.exclusive_access();
+    timers.push(TimerCondVar {
+        expire_ms,
+        task,
+    });
+}
+
+pub fn check_timer() {
+    let current_ms = get_time_ms();
+    let mut timers = TIMERS.exclusive_access();
+    while let Some(timer) = timers.peek() {
+        if timer.expire_ms <= current_ms {
+            add_task(Arc::clone(&timer.task)); 
+            drop(timer);
+            timers.pop();
+        } else { break; }
+    }
+}
diff --git a/os/src/trap/mod.rs b/os/src/trap/mod.rs
index 7f214195..5fd95f5b 100644
--- a/os/src/trap/mod.rs
+++ b/os/src/trap/mod.rs
@@ -20,7 +20,7 @@ use crate::task::{
     current_trap_cx,
     current_trap_cx_user_va,
 };
-use crate::timer::set_next_trigger;
+use crate::timer::{set_next_trigger, check_timer};
 use crate::config::TRAMPOLINE;
 
 global_asm!(include_str!("trap.S"));
@@ -83,6 +83,7 @@ pub fn trap_handler() -> ! {
         }
         Trap::Interrupt(Interrupt::SupervisorTimer) => {
             set_next_trigger();
+            check_timer();
             suspend_current_and_run_next();
         }
         _ => {
diff --git a/user/src/lib.rs b/user/src/lib.rs
index 7d3dd70b..356360e9 100644
--- a/user/src/lib.rs
+++ b/user/src/lib.rs
@@ -100,11 +100,8 @@ pub fn waitpid(pid: usize, exit_code: &mut i32) -> isize {
         }
     }
 }
-pub fn sleep(period_ms: usize) {
-    let start = sys_get_time();
-    while sys_get_time() < start + period_ms as isize {
-        sys_yield();
-    }
+pub fn sleep(sleep_ms: usize) {
+    sys_sleep(sleep_ms);
 }
 
 pub fn thread_create(entry: usize) -> isize { sys_thread_create(entry) }
diff --git a/user/src/syscall.rs b/user/src/syscall.rs
index df6f4cb8..793b36dd 100644
--- a/user/src/syscall.rs
+++ b/user/src/syscall.rs
@@ -5,6 +5,7 @@ const SYSCALL_PIPE: usize = 59;
 const SYSCALL_READ: usize = 63;
 const SYSCALL_WRITE: usize = 64;
 const SYSCALL_EXIT: usize = 93;
+const SYSCALL_SLEEP: usize = 101;
 const SYSCALL_YIELD: usize = 124;
 const SYSCALL_GET_TIME: usize = 169;
 const SYSCALL_GETPID: usize = 172;
@@ -61,6 +62,10 @@ pub fn sys_exit(exit_code: i32) -> ! {
     panic!("sys_exit never returns!");
 }
 
+pub fn sys_sleep(sleep_ms: usize) -> isize {
+    syscall(SYSCALL_SLEEP, [sleep_ms, 0, 0])
+}
+
 pub fn sys_yield() -> isize {
     syscall(SYSCALL_YIELD, [0, 0, 0])
 }