The total dose effect of 0.8 μm SOI NMOS device with H-gate is analyzed. The device is exposed to 60Co γ-ray at low dose rate. The result shows that the irradiation effect is more serious at low dose rate for the same total irradiation dose. The threshold voltage shift in on state is lower than in off state. Irradiation leads to the increased drain voltage VD of kink effect. Because the number of effective interface traps varies with gate bias, interface trap has different influences on sub-threshold slope and transconductance.