Radical nitridation of ultra-thin thermal oxides of SiC was performed to improve the SiC MOS interface properties. The gate oxides of MOSFETs were formed by the following sequence: dry oxidation to grow 4nm-thick thermal oxides, nitridation of thermal oxides by nitrogen radical irradiation, and deposition of 80 nm-thick SiO2 by plasma CVD method. For 4 nm-thick thermal oxides, nitrogen was introduced into the SiO2/SiC interface at 3 at% by radical nitridation, which was confirmed by the XPS measurements. By adopting radical nitridation of the ultra-thin thermal oxides, the peak field-effect channel mobility was increased from 11 to 31 cm(2)/VS for 4H-SiC and from 60 to 66 cm(2)/Vs for 6H-SiC MOSFETs. The threshold voltage was reduced from 5.4 to 4.7 V for 4H-SiC and 2.9 to 2.5 V for 6H-SiC due to the reduction of trapped electrons at interface states.