Nitrogen (N) radical irradiation after dry thermal oxidation of 4H- and 614-SiC was found to be effective to improve the MOSFET performance. XPS measurements revealed that N atoms were introduced into the SiO2/SiC interface at approximately 1 at% and formed Si-N bonds. Drain current of MOSFETs irradiated N radicals was increased compared to MOSFETs without N radical irradiation at the same bias condition. Field-effect channel mobility was increased from 1.2 cm(2)/Vs to 2.9 cm(2)/Vs for 4H-SiC and 22 cm(2)/Vs to 33 cm(2)/Vs for 6H-SiC MOSFETs by adding nitrogen radical irradiation process, owing to reduction of interface state density. The improvement of SiO2/SiC interface properties was achieved due to formation of Si-N bonds at the interface by N radical irradiation.