With the improved performance of IT devices like car microprocessors, the heat generated in the electric devices are being increased in amount. The increased heating value degrades the performance of devices and circuits, which will affect the durability and reliability of products eventually by causing their failure. Therefore, a study on the cooling performance improvement of heat sinks is essential to decrease the temperatures of the field effect transistors (FETs) in cars. In this study, numerical analysis and experimental tests were carried out to develop a new heat sink design for a different FET material, heat conductive resin. Without the heat sink, the new FET had a temperature distribution of 70–90 Celsius, which was measured in the laboratory environment similar to the real chamber within a car. Based on the numerical and experimental test and analysis of the FET new heat sink system, the improved shape and array of the fins are developed that meet the target temperature of the FET. For heat sinks, efficient cooling design and reduced manufacturing costs are derived from the comparison with those of old heat sink material, Aluminum.