This paper presents the heat transfer characteristics of the internal heat exchanger (IHX) for $CO_2$ heat pump system. The influence on the IHX length, the mass flow rate, the shape of IHX, the operating condition, and the oil concentration was investigated under a cooling condition. Four kinds of IHX with a coaxial type and a micro-channel type, a mass flow meter, a pump, and a measurement system. With increasing of the IHX length, the capacity, the effectiveness, and the pressure drop increased. For the mass flow rate, the capacity of micro-channel IHX are higher about 2 times than those of coaxial IHX. The pressure drop was larger at cold-side than at hot-side. In the transcritical $CO_2$ cycle, system performance is very sensitive to the IHX design. Design parameters are closely related with the capacity and the pressure drop of $CO_2$ heat pump system. Along the operating condition, the performance of $CO_2$ IHXs is different remarkably. For oil concentration 1, 3, 5%, the capacity decreases and the pressure drop increased, as compared with oil concentration 0%.