In this paper, the electrical properties of pre- and post-rapid thermal annealed 4H SiC-based gate oxide grown in 10% nitrous oxide (N2O) and in dry oxygen have been investigated, compared, and reported for the first time. After treating the nitrided gate oxide in rapid thermal annealing (RTA), oxide breakdown characteristic has been improved significantly. This improvement has been attributed to the reduction of SiC–SiO2 interface-trap density and the generation of positive oxide charge, acting as an electron-trapping center. However, deleterious effects have been observed in non-nitrided oxide after subjected to the same RTA treatment. The differences in oxide-breakdown strength of these oxides have been explained and modeled.