Coating of Ti-6Al-4V alloy with Ti-O film by reactive DC sputtering in Ar-O-2, gas mixtures, in order to improve the biocompatibility of the alloy, was examined. The effects of oxygen content in the gas mixtures on the formation of Ti-O film were investigated. Under visual observation, the films deposited under various oxygen contents appeared to be uniform and adhesive. Under SEM, the surface of the Ti-O films had fine particles dispersed on a smooth accumulated deposit. Based on observation of the accumulated deposit, we expected that the addition of oxygen to sputter gas might improve the adhesion between the deposited films and the alloy substrates in this study. By analyzing the surface roughness of obtained films, we found that the addition of oxygen to sputter gas improved topographic fidelity of the film to the surface of the alloy substrate onto which it was deposited and that the topographic fidelity depended on the oxygen content of the gas mixture. According to AES, the Ti/O ratio in depth direction was nearly constant in each of the films and oxygen concentration increased with increasing oxygen content of the sputter gas. The Vickers hardness of the films increased almost linearly with increasing oxygen content, and the maximum hardness reached over Hv1600 for the him deposited under the oxygen flow rate of 3.0 mL/min. This confirmed that coating of Ti-6Al-4V alloy with Ti-O film improved the hardness of the alloy. Further, the formation of titanium oxides or suboxides in the Ti-O film was assumed because of its high hardness and high oxygen concentration.