Oxygen deficient (O-deficient) anatase thin films with high photoreactivity under UV-light irradiation were fabricated by post-annealing partially nitrided anatase thin films, prepared by heat-treatment of precursor films involving a Ti complex of EDTA on an FTO glass substrate at 500 degrees C for 30 min in an Ar gas flow, in air at 500 degrees C for 5-30 min. The anatase structure of the transparent thin films was characterized by using XRD and Raman spectra. The O/Ti peak area ratio determined by using XPS of the anatase film having the highest photoreactivity, which was evaluated according to the decoloration rate of methylene blue in an aqueous solution, was 1.5. The photoreactivity of the film was 2.1 times higher than that prepared by using the sol-gel method, with an O/Ti ratio of 1.7. The thin film with the highest photoreactivity indicated the smallest refractive index, 1.99.