Electrical behavior of commercial off-the-shelf normally-off GaN power transistors under heavy ion irradiation is presented based on technology computer aided design numerical simulation in order to better understand the mechanism of single event effects (SEEs) in these devices. First, the worst case has been defined from the single event transient mechanism. Then, the decrease in the electric field observed after irradiation and the traps effect have been addressed. Finally, possible mechanisms of SEE in these devices under heavy ion are proposed.