Based on 0.8 μm process, in the paper we investigate the total dose irradiation effects of SOI NMOS devices under different bias conditions. The devices are exposed to 60Co γ ray at a dose rate of 50 rad (Si)/s. In higher gate bias condition, the drain leakage current increases because more positive charges are trapped in the buried oxide. When the applied gate voltage is larger than the threshold voltage, its drain current of the front gate in ID-VG characteristic suddenly increases and the body current presents a unique upside down bell shape.