Generally when Power MOSFET is operated, a heat sink is attached to it to emit heat caused by the operation. As the surface area of a heat sink is smaller, the thermal impedance is larger, which causes a negative influence on the characteristics of the chips and the devices and shortens the lifespan of them. In this experiment, we``ve compared and analysed different effects of heat sinks with 5 different surface areas on the characteristics of Thermal Transient when they are applied respectively.