Abstract: The ionizing radiation effects on MOS transistors with different device sizes were studied. The test devices were designed and fabricated in a commercial 0.6 m standard bulk CMOS process. Device parameters were monitored before and after ⁶⁰Co γ-rays irradiation with total dose of 9.6 kGy(Si). The experiment results show that the threshold voltage shift after γ-ray irradiation is not sensitive to W/L in both NMOS and PMOS devices. The increases of leakage between source and drain induced by irradiation are different in different sized NMOSs. For the same channel length NMOSs, smaller W/L causes larger leakage.