Coating of Ti-6Al-4V alloy substrates with Ti-O films was carried out by reactive DC sputtering in Ar-O mixtures to improve the dental applicability of the alloy. The effects of discharge voltage (200-500 V) on Ti-O film formation were investigated. The Ti-O films deposited under various discharge voltages appeared to be uniform and adhesive. while their color tone and Luster varied with the voltage. The films obtained under a voltage higher than 250 V were hard and durable. and the ones obtained under a voltage higher than 280 V were compact with less defects. The Ti/O ratio of each film was nearly constant in depth direction and its oxygen concentration depended on the voltage, which was detected by Auger electron spectroscopy (AES), On the basis of X-lay diffractometry (XRD), it was concluded that the suboxides such as Ti4O7 and Ti6O11 were formed in the films and that Ti2O was also formed under a voltage higher than 400 V. The hardness of the films increased with the increase in voltage of up to 400 V. but slightly decreased at a higher voltage range. The maximum hardness higher than Hv2000 was noteworthy.