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 μm 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 y-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 NMOS devices. For the same channel length NMOS devices, smaller W/L causes larger leakage.