Abstract: Total dose radiation hard NMOSFETs were fabricated on SIMOX substrate. N₂O annealed H2-O₂ grown dielectric was used to harden the front gate. The body region was doped high to increase the back gate threshold voltage. And a H-gate structure was adopted to eliminate the lateral MOS transistor. The results show that the characteristic of the NMOSFETs does not change much after theradiation of 1E6rad（Si）.