A high performance photocatalytic TiO2 thin film was successfully obtained by reactive DC magnetron sputtering. The film was deposited onto SiO2-coated glass at a substrate temperature of 220 degreesC using a titanium metal target in O-2 100% atmosphere. The film showed good uniformity of thickness in a large area with the optical transmittance of similar to 80% in the visible region. The decomposition ability of acetaldehyde (CH3CHO) of the film under UV irradiation was almost the same as that of the sol-gel-derived TiO2 thin film but the sputtered film showed a much higher mechanical durability. The characterization of the films was carried out using XRD, SEM, AFM, XPS and SIMS, and the electronic structures of the films were calculated using a first-principle calculation method based on the density functional theory. It was found that the amount of incorporated O-18 into the film was larger for the films with lower photocatalytic activity when the films were annealed in O-18(2)/N-2 atmosphere. This result indicates that the amount of oxygen vacancies, which were occupied by incorporated O-18, was larger for the films with lower photocatalytic activity. Furthermore, the introduction of structural defects associated with oxygen vacancies was found to create some energy levels around the mid-gap, indicating that they could work as recombination centers of photo-induced holes and electrons, causing the decrease in photocatalytic activity. Therefore, the decrease in the structural defects associated with oxygen vacancies is important for improving the photocatalytic activity of the films.