A 0.18 m MOSFET with shallow trench isolation is exposed to a -ray radiation. The parameters such as off-state leakage current, threshold voltage, transconductance, gate leakage current, and subthreshold slope are analyzed for pre- and post-irradiation. By introducing constant sheet charges at the shallow trench isolation oxide sidewall, good agreement between 3D simulation and experiment result is demonstrated. We believe that the thin gate oxide is insensitive to radiation, and the radiation induced charge trapping in the shallow trench isolation still leads to macroscopic effects such as drain-to-source leakage current, ultimately limiting the tolerance of CMOS circuits.