diff --git a/src/core/include/scene/unit.hpp b/src/core/include/scene/unit.hpp
index 289c0bd..41ad459 100644
--- a/src/core/include/scene/unit.hpp
+++ b/src/core/include/scene/unit.hpp
@@ -207,7 +207,87 @@ namespace gkit::scene {
 
     private:
         std::atomic<bool> process_enabled = true; // process enabled flag
-        std::atomic<bool> ready_to_drop  = false; // drop flag(mark as dead)
+        std::atomic<bool> ready_to_drop = false;  // drop flag(mark as dead)
+
+    public:
+        // iterator
+        class iterator {
+        public:
+            using iterator_category = std::bidirectional_iterator_tag;
+            using value_type = Unit;
+            using difference_type = std::ptrdiff_t;
+            using pointer = Unit*;
+            using reference = Unit&;
+
+        public:
+            iterator(Unit* owner, size_t pos);
+            auto operator*() -> reference const;
+            auto operator->() -> pointer const;
+            auto operator++() -> iterator&;
+            auto operator++(int) -> iterator;
+            auto operator--() -> iterator&;
+            auto operator--(int) -> iterator;
+            auto operator==(const iterator& other) const -> bool;
+            auto operator!=(const iterator& other) const -> bool;
+
+        private:
+            Unit* m_owner;
+            size_t m_pos;
+            friend class Unit;
+        };
+
+        // Why this part didn't use auto, because it need to have a const_iterator use
+        // but auto could not allow two same function but with different return
+        // but begin() and end() need those two return
+        // So I will not change it
+        auto begin() -> iterator;
+        auto end() -> iterator;
+
+    public:
+        // Next, we are going to write the const implementation of the iterator.
+        class const_iterator {
+        public:
+            using iterator_category = std::bidirectional_iterator_tag;
+            using value_type = const Unit;
+            using difference_type = std::ptrdiff_t;
+            using pointer = const Unit*;
+            using reference = const Unit&;
+
+            const_iterator(const Unit* owner, size_t pos);
+            auto operator*() -> reference const;
+            auto operator->() -> pointer const;
+            auto operator++() -> const_iterator&;
+            auto operator++(int) -> const_iterator;
+            auto operator--() -> const_iterator&;
+            auto operator--(int) -> const_iterator;
+            auto operator==(const const_iterator& other) const -> bool;
+            auto operator!=(const const_iterator& other) const -> bool;
+
+        private:
+            const Unit* m_owner;
+            size_t m_pos;
+        };
+
+        auto begin() const -> const_iterator;
+        auto end() const -> const_iterator;
+
+        auto cbegin() const -> const_iterator;
+        auto cend() const -> const_iterator;
+
+    public:
+        // This is a reverse iterator, implemented using std::reverse_iterator.
+        using reverse_iterator = std::reverse_iterator<iterator>;
+        using const_reverse_iterator = std::reverse_iterator<const_iterator>;
+
+        auto rbegin() -> reverse_iterator;
+        auto rend() -> reverse_iterator;
+
+        auto rbegin() const -> const_reverse_iterator;
+        auto rend() const -> const_reverse_iterator;
+
+        auto crbegin() const -> const_reverse_iterator;
+        auto crend() const -> const_reverse_iterator;
+
     }; // class Unit
 
     template <IsUnitExtend T>
diff --git a/src/core/scene/unit.cpp b/src/core/scene/unit.cpp
index 63ba2c7..2c21eaa 100644
--- a/src/core/scene/unit.cpp
+++ b/src/core/scene/unit.cpp
@@ -184,3 +184,101 @@ auto gkit::scene::Unit::get_parent<gkit::scene::Unit>() noexcept -> std::optiona
     if (parent == nullptr) return std::nullopt;
     return std::ref(*parent);
 }
+
+
+// iterator part use
+gkit::scene::Unit::iterator::iterator(Unit* owner, size_t pos) : m_owner(owner), m_pos(pos) {}
+auto gkit::scene::Unit::iterator::operator*() -> reference const {
+auto child_opt = m_owner->get_available_child(static_cast<uint32_t>(m_pos));
+    return **child_opt;
+}
+auto gkit::scene::Unit::iterator::operator->() -> pointer const {
+    auto child_opt = m_owner->get_available_child(static_cast<uint32_t>(m_pos));
+    return *child_opt;
+}
+auto gkit::scene::Unit::iterator::operator++() -> iterator& {
+    ++m_pos;
+    return *this;
+}
+auto gkit::scene::Unit::iterator::operator++(int) -> iterator {
+    iterator tmp = *this;
+    ++(*this);
+    return tmp;
+}
+auto gkit::scene::Unit::iterator::operator--() -> iterator& {
+    --m_pos;
+    return *this;
+}
+auto gkit::scene::Unit::iterator::operator--(int) -> iterator {
+    iterator tmp = *this;
+    --(*this);
+    return tmp;
+}
+auto gkit::scene::Unit::iterator::operator==(const iterator& other) const -> bool  { return m_owner == other.m_owner && m_pos == other.m_pos; }
+auto gkit::scene::Unit::iterator::operator!=(const iterator& other) const -> bool  { return !(*this == other); }
+
+auto gkit::scene::Unit::begin() -> iterator{
+    return iterator(this, 0);
+}
+
+auto gkit::scene::Unit::end() -> iterator{
+    return iterator(this, active_index_cache.size());
+}
+
+// now is const_iterator use
+gkit::scene::Unit::const_iterator::const_iterator(const Unit* owner, size_t pos) : m_owner(owner), m_pos(pos) {}
+
+auto gkit::scene::Unit::const_iterator::operator*() -> reference const {
+    auto child_opt = const_cast<Unit*>(m_owner)->get_available_child(static_cast<uint32_t>(m_pos));
+    return **child_opt;
+}
+
+auto gkit::scene::Unit::const_iterator::operator->() -> pointer const {
+    auto child_opt = const_cast<Unit*>(m_owner)->get_available_child(static_cast<uint32_t>(m_pos));
+    return *child_opt;
+}
+
+auto gkit::scene::Unit::const_iterator::operator++() -> const_iterator& {
+    ++m_pos;
+    return *this;
+}
+auto gkit::scene::Unit::const_iterator::operator++(int) -> const_iterator {
+    const_iterator tmp = *this;
+    ++(*this);
+    return tmp;
+}
+auto gkit::scene::Unit::const_iterator::operator--() -> const_iterator& {
+    --m_pos;
+    return *this;
+}
+auto gkit::scene::Unit::const_iterator::operator--(int) -> const_iterator {
+    const_iterator tmp = *this;
+    --(*this);
+    return tmp;
+}
+
+auto gkit::scene::Unit::const_iterator::operator==(const const_iterator& other) const -> bool { return m_owner == other.m_owner && m_pos == other.m_pos; }
+auto gkit::scene::Unit::const_iterator::operator!=(const const_iterator& other) const -> bool { return !(*this == other); }
+
+auto gkit::scene::Unit::begin() const -> const_iterator {
+    const_cast<Unit*>(this);
+    return const_iterator(this, 0);
+}
+
+auto gkit::scene::Unit::end() const -> const_iterator {
+    const_cast<Unit*>(this);
+    return const_iterator(this, active_index_cache.size());
+}
+
+auto gkit::scene::Unit::cbegin() const -> const_iterator { return begin(); }
+auto gkit::scene::Unit::cend() const -> const_iterator { return end(); }
+
+// This is a reverse iterator, implemented using std::reverse_iterator.
+using reverse_iterator = std::reverse_iterator<gkit::scene::Unit::iterator>;
+using const_reverse_iterator = std::reverse_iterator<gkit::scene::Unit::const_iterator>;
+auto gkit::scene::Unit::rbegin() -> reverse_iterator { return reverse_iterator(end()); }
+auto gkit::scene::Unit::rend() -> reverse_iterator { return reverse_iterator(begin()); }
+auto gkit::scene::Unit::rbegin() const -> const_reverse_iterator { return const_reverse_iterator(end()); }
+auto gkit::scene::Unit::rend() const -> const_reverse_iterator { return const_reverse_iterator(begin()); }
+auto gkit::scene::Unit::crbegin() const -> const_reverse_iterator { return rbegin(); }
+auto gkit::scene::Unit::crend() const -> const_reverse_iterator { return rend(); }
\ No newline at end of file