Abstract: To find out the law that the geometric parameters of the rectangular fin heat sink affect heat radiation,convective heat radiation computation is carried out with the thermal simulation analysis software Flotherm for the rectangular fin heat sink models under different structure parameters in this paper．Effects of the base plate parameters and the fin parameters on the thermal performance are discussed by the computation result comparison and analysis of the temperatures and thermal resistances under different models．The simulation results show that the thermal resistance of the heat sink can be reduced effectively by changing the height，length and gap of fins．These geometric parameters can be used as thermal design variables in optimization design of the heat sink．