The main purpose of this research is to utilize the FEMLAB multiphysics software as to analyze the thermal stress of Cu/Diamond/Cu three-layered heat spreader device. The alteration of film thickness under different temperatures(allowing the device to increase from room temperature of 27¢XC to 100¢XC, 200¢XC and 300¢XC), with upper Cu layer of 2µm, altered middle diamond layer(20µm, 50µm 100µm), and lower Cu layer(100µm, 200µm, 300µm)(assuming that there is no residual stress produced after the manufacture of heat spreader device), is regarded as the conditional parameter of heat spreader device in analyzing the deformation rate and stress variation produced. After simulation of the research, it was found out that a maximum shear stress is attained under a fixed temperature load. And, when the film thickness is altered and under a fixed thick of Cu layer, the shear stress will become bigger due to the thickness of the diamond layer increases, when the diamond layer is 50-70µm, stress increases slowly; but it is considered to be the greatest effect when the heat transfer of diamond is at 100µm; thus selection of 70µm would be a better option in this paper suggestion, and that the alteration of the thickness of lower Cu layer will not cause great effect to max shear stress.) After stabilizing the thickness of upper Cu layer and middle diamond layer, the maximum deformation rate will become smaller when the thickness of lower Cu layer increases.