diff --git a/include/pocketpy/interpreter/gc.h b/include/pocketpy/interpreter/gc.h
index 1c5e31cc..c4e420d0 100644
--- a/include/pocketpy/interpreter/gc.h
+++ b/include/pocketpy/interpreter/gc.h
@@ -13,8 +13,7 @@ typedef struct pk_ManagedHeap{
     int gc_counter;
     pk_VM* vm;
 
-    void (*_gc_on_delete)(pk_VM*, PyObject*);
-    void (*_gc_marker_ex)(pk_VM*);
+    void (*gc_on_delete)(pk_VM*, PyObject*);
 } pk_ManagedHeap;
 
 void pk_ManagedHeap__ctor(pk_ManagedHeap* self, pk_VM* vm);
diff --git a/src/interpreter/gc.c b/src/interpreter/gc.c
index 77ebf2d5..656aeb38 100644
--- a/src/interpreter/gc.c
+++ b/src/interpreter/gc.c
@@ -10,8 +10,7 @@ void pk_ManagedHeap__ctor(pk_ManagedHeap *self, pk_VM *vm){
     self->gc_counter = 0;
     self->vm = vm;
 
-    self->_gc_on_delete = NULL;
-    self->_gc_marker_ex = NULL;
+    self->gc_on_delete = NULL;
 }
 
 void pk_ManagedHeap__dtor(pk_ManagedHeap *self){
@@ -54,8 +53,8 @@ int pk_ManagedHeap__sweep(pk_ManagedHeap *self){
             obj->gc_marked = false;
             c11_vector__push(PyObject*, &alive, obj);
         } else {
-            if(self->_gc_on_delete){
-                self->_gc_on_delete(self->vm, obj);
+            if(self->gc_on_delete){
+                self->gc_on_delete(self->vm, obj);
             }
             PyObject__delete(obj);
         }
diff --git a/src/interpreter/vm.c b/src/interpreter/vm.c
index 739db762..9a24cfa3 100644
--- a/src/interpreter/vm.c
+++ b/src/interpreter/vm.c
@@ -509,19 +509,60 @@ void PyObject__delete(PyObject* self) {
     }
 }
 
+static void mark_object(PyObject* obj);
+
+static void mark_value(py_TValue* val) {
+    if(val->is_ptr) mark_object(val->_obj);
+}
+
+static void mark_object(PyObject* obj) {
+    if(obj->gc_marked) return;
+    obj->gc_marked = true;
+
+    // list is a special case
+    if(obj->type == tp_list) {
+        c11_vector* vec = PyObject__userdata(obj);
+        c11__foreach(py_TValue, vec, p) mark_value(p);
+        return;
+    }
+
+    if(obj->slots > 0) {
+        py_TValue* p = PyObject__slots(obj);
+        for(int i = 0; i < obj->slots; i++)
+            mark_value(p + i);
+        return;
+    }
+
+    if(obj->slots == -1) {
+        pk_NameDict* dict = PyObject__dict(obj);
+        for(int j = 0; j < dict->count; j++) {
+            pk_NameDict_KV* kv = c11__at(pk_NameDict_KV, dict, j);
+            mark_value(&kv->value);
+        }
+        return;
+    }
+}
+
 void pk_ManagedHeap__mark(pk_ManagedHeap* self) {
-    // for(int i=0; i<self->no_gc.count; i++){
-    //     PyObject* obj = c11__getitem(PyObject*, &self->no_gc, i);
-    //     vm->__obj_gc_mark(obj);
-    // }
-    // vm->callstack.apply([vm](Frame& frame) {
-    //     frame._gc_mark(vm);
-    // });
-    // vm->obj_gc_mark(vm->__last_exception);
-    // vm->obj_gc_mark(vm->__curr_class);
-    // vm->obj_gc_mark(vm->__c.error);
-    // vm->__stack_gc_mark(vm->s_data.begin(), vm->s_data.end());
-    // if(self->_gc_marker_ex) self->_gc_marker_ex((pkpy_VM*)vm);
+    pk_VM* vm = self->vm;
+    // mark heap objects
+    for(int i = 0; i < self->no_gc.count; i++) {
+        PyObject* obj = c11__getitem(PyObject*, &self->no_gc, i);
+        mark_object(obj);
+    }
+    // mark value stack
+    for(py_TValue* p = vm->stack.begin; p != vm->stack.end; p++) {
+        mark_value(p);
+    }
+
+    // mark vm's registers
+    mark_value(&vm->last_retval);
+    mark_value(&vm->last_exception);
+    for(int i = 0; i < c11__count_array(vm->reg); i++) {
+        mark_value(&vm->reg[i]);
+    }
+
+    mark_value(&vm->__curr_class);
 }
 
 void pk_print_stack(pk_VM* self, Frame* frame, Bytecode byte) {