The behavior of medium voltage commercial power MOSFETs, first degraded with increasing c-rays doses and subsequently irradiated with heavy ions, is presented. It is shown that the degradation of the gate oxide caused by the c irradiation severely corrupt the SEE robustness and drastically modify the physical behavior of the device under test after the impact of a heavy ion. A decrease of the critical voltages at which destructive burnouts and gate ruptures appear has been detected in all devices previously irradiated with c-rays, the amount of the critical voltage reduction is strictly related to the amount of the absorbed c-rays dose. Furthermore, at the failure voltage, the behavior of the device is affected by the conduction of a current through the gate oxide. Moreover the SEGR of the device appears at lower voltages due to the reduction of the Fowler–Nordheim limit in the c-irradiated devices.