RECENTLY POWER MOSFETS have been used for satellite power supplies. NASDA has developed such a Power MOSFET for the space projects. It has a metal-type package, and the die is attached by Au-Si alloy to achieve high temperature operation (Tj=200°C). The fabricated device failure was detected thermal resistance tests after high temperature storage test at 200°C for 1000 hours. This failure is caused by the die peeling between the Au-Si eutectic alloy layer and the Ni plating surface on the Cu-W substrate. In this paper, we describe the failure mechanism and improvements in the package design and fabrication process. The newly developed power MOSFET for space use exhibits good performance in the high temperature storage test.