The effects of pre-irradiation high electric field and elevated-temperature bias stressing on radiation response of power VDMOSFETs have been investigated. Compared to unstressed devices, larger irradiation induced threshold voltage shift and mobility reduction in high electric field stressed devices have been observed, clearly demonstrating inapplicability of electrical stressing for radiation hardening of power MOSFETs. On the other hand, larger irradiation induced threshold voltage shift in elevated-temperature bias stressed, and more considerable mobility reduction in unstressed devices have been observed, confirming the necessity of performing the radiation qualification testing after the reliability screening of these devices. The underlying changes of gate oxide-trapped charge and interface trap densities have been calculated and analysed in terms of the mechanisms responsible for pre-irradiation stress effects.