Abstract: A two dimensional numerical simulation for NMOSFET with lateral parasitic transistor is presented. The influences of three key parameters such as the doping level, gate bias during irradiation, the bird's beak shape on NMOSFET are studied for total dose radiation hardening performance. The result shows that increasing the doping level below the bird's beak region and field oxide region, lower gate bias and reduction of the bird's slope are helpful to reduce lateral leakage current and enhance rad tolerant ability.