Transparent, amorphous, surface smooth, and hard aluminum oxide thin films were deposited on Si (100) and quartz substrates by an off-plane filtered cathodic vacuum arc (FCVA) system. We systematically studied the optical properties, such as transmittance and optical constants, of the aluminum oxide thin films deposited under various oxygen partial pressures. Experimental results show that the optical properties for aluminum oxide thin films are strongly dependent on oxygen partial pressure; the higher oxygen partial pressure causes much more oxygen to incorporate into the film thus inducing more modification of film properties. At proper oxygen partial pressure, the stoichiometric alumina film is obtained, which is smooth and dense. Additionally, it also displays good optical properties with a refractive index of similar to1.7 at 550 nm. The properties exhibited by the alumina thin films manifest the potential applications for alumina thin films in optical coatings and for FCVA technology in deposition of metal oxide optical films. (C) 2003 American Vacuum Society.