Thermally grown $SiO_2$ films on silicon have been thermally nitrided in the $NH_3$ gas ambient and their properties have been investigated by analyzing the AES data and the results of the I-V and the C-V measurements. The Auger depth profile show that the nitrogen-rich layers are formed at the nitrided oxide film surface and near the $SiO_2$-Si interface. The higher the nitridation temperature is, the larger the refractive index of nitroxide film is. And the thinner the oxide film to be nitrided for the same nitridation temperature is, the larger the refractive index of nitroxide film is. When thin $SiO_2$ film is thermally nitrided, the I-V characteristics show the Fowler-Nordheim conduction fashion. Flatband voltages of $SiO_2$ films are shifted by nitridation, due to the fixed charge formation.