High quality SOI (silicon on insulator) materials were fabricated using highly selective and self-stopping full isolation by porous oxidized silicon (FIPOS) technology, then n-channel MOSFETs were successfully fabricated on the wafers. The discrete NMOS/FIPOS transistors with different width/length ratios were subjected to ⁶⁰Co γ-ray radiation and their subthreshold-current I_d-V_g curves were measured as a function of radiation dose. The transistors remain function after 5×10³ Gy (Si) radiation, but the threshold voltage has significantly shifted to negative and its major contribution comes from the radiation-induced trapped oxide charges. The threshold voltage shift turns back during the room-temperature annealing due to partially detrapping of the trapped holes in the oxide layers. The parasitic back-channel effects of the n-channel MOSFETs/FIPOS are negligible and their hardiness to total dose radiation is similar to that of bulk MOSFETs.