High temperature (1250° C) NO annealing was performed for deposited oxide (SiO2) and oxynitride (SiON) films on n-type 4H-SiC. Interface state density of SiO2 samples was dramatically reduced (one order of magnitude) by NO annealing compared to N-2 annealing, resulting in 1 x 10(11) cm(-2) eV(-1) at 0.2 eV below the conduction band edge. In contrast, that of NO annealed SiON samples showed only 30 % decrease and was still in the range of 1 x 10(12) cm(-2)eV(-1). These different effects of NO annealing against SiO2 and SiON are probably due to different reaction mechanism at the interface. Breakdown field of SiO2 samples annealed in NO was as high as 10 MV/cm. Barrier height of this sample was 2.86eV, which is close to the ideal value.