Abstract: This paper described the thermal design of metaloxidesemiconductor fieldeffect transistor(MOSFET) which is typically applied in highfrequency power supply. After introducing the thermal parameters of MOSFET, the thermal risistance networks for single and couple pieces were established respectively, and further temperature formulations at each interface were deduced. Based on the above work, a suitable heat sink was figured out to solve the thermal problem in a highfrequence power supply which contains four MOSFETs compactively fix in a PCB. Finally, the performance of heat sink was verified with the help of computational fluid dynamics（CFD）software ICEPAK, and the thermal risistance networks are proved to be practical in engineering application