The effect of oxygen addition to TiN was investigated by producing oxynitride films with the arc ion plating method. These titanium oxynitride films were identified to be NaCl(B1) structure by XRD measurements with θ-20 and glancing angle methods. By increasing the content of oxygen in the deposited films, the preferential orientation of the films changed from the <111> direction to the <200> direction, and the existence of titanium ions with a higher valence, such as Ti 3+ and Ti 4+ as well as divalent titanium, in the deposited films was indicated by the change of Ti 2p photoelectron spectra. From the result on Vickers hardness, it was also indicated that suitable addition of oxygen to TiN is preferable to improve the hardness of the TiN films.