We have investigated the effects of nitrogen (N) and oxygen (0) radical treatment on interface properties of alpha-SiC MOS structure. Standard Hi-Lo C-V technique was used to determine the SiO2/SiC interface state density. From the Hi-Lo C-V analysis, the density of interface states was reduced by irradiation of N radicals. This tendency was observed more significantly in the case of the thinner oxide film. From the results, the reduction of interface state density could depend on initial SiO2/SiC interface state properties and the amount of N radicals, which reach into the SiO2/SiC interface. Upon irradiation by 0 radicals, the breakdown electric field of the gate oxide film decreases and the interface state density increases.