Cr thin films with O added are deposited on sapphire substrate by DC sputtering and are nitrided in NH3 atmosphere between 300 and 900 °C for various times. X-ray diffraction results show that nitridation begins at 500 °C, forming CrN and Cr2N. Cr oxides of Cr2O3 are formed at 600 °C. And, at temperatures higher than 900 °C, the intermediate materials of Cr2N and Cr2O3 disappear and CrN is dominant. The atomic concentration ratios of Cr and O are 77% and 23%, respectively, over the entire thickness of as-deposited Cr thin film. In the sample nitrided at 600 °C, a CrN layer in which O is substituted with N is formed from the surface to 90 nm, and the concentrations of Cr and N in the layer are 60% and 40%, respectively. For this reason, CrN and Cr2N are distributed in the CrN region, where O is substituted with N by nitridation, and Cr oxynitrides are formed in the region below this. The nitridation process is controlled by inter-diffusion of O and N and the parabolic growth law, with activation energy of 0.69 eV.