We investigated the thermal cycle properties of Cu and Al circuit substrates using FEM analysis. The thermal stress in AIN for the Cu circuit substrate is larger than the Al circuit substrate and it increases as the thermal cycling proceeds. The Mises strain in the solder which joins the Si chip on the Cu circuit substrate is larger than the Al circuit substrate, and the fatigue life of the solder on the Al circuit substrate is estimated to be about 1400 cycles longer than the Cu circuit substrate. These results are due to the difference of the work hardening rate of Cu and Al. Cu and Al circuits have a plastic deformation during the thermal cycling, thus, Cu becomes harder due to its large work hardening. On the other hand, since the work hardening rate of Al is much less than Cu, the flow stress of Al does not increase with thermal cycling. It is suggested that using Cu lowers the reliability of power modules.