Post-oxidation annealing (POA) of the SiO2/4H-SiC in Ar diluted NO ambient has been performed as a function of the NO concentration for providing better understanding of the nitridation. Experiments were conducted by using a horizontal cold-wall oxidation furnace, which consists of infrared lamps coupled with quartz reaction tube, in order to minimize the thermal decomposition of NO molecules before reaching sample surface. The interface trapped density (D-it) is found to be reduced even by an annealing in 2% NO ambient with compared to that without NO annealing. Meanwhile, the D-it in case of 2% shows higher densities than those of 5% and 20% under the same annealing temperature and time, indicating impacts of the nitridation depends on the NO concentration. Hot-carrier tolerance of the n-type MOS capacitors was minimized with the NO concentration of 2-5%. Inversion channel mobilities of the fabricated MOSFETs with the nitridation depend on NO concentration, and it increases with increasing the NO concentration and saturates around 5%.