High quality gate insulator film formation on 4H-SiC substrate is demonstrated. The insulator films were formed by the PECVD and radical oxynitridation using microwave-excited high-density plasma with NO gas at low temperature. The oxide fixed charge and the interface trap density can be dramatically reduced by NO gas radical oxynitridation after the oxide film formed by the PECVD compared with by direct oxynitridation on 4H-SiC. SIMS profiles show carbon profile in these fabricated gate insulator films. It is confirmed that the electrical property is improved as the amount of remaining carbon in the insulator film decreases.