Nitrogen-doped titanium oxide (N-TiO2) films deposited on unheated glass slides at various flow rates of N2 in an atmosphere of Ar and O2 gas mixture by direct current sputtering technique were investigated. The Ti2p, O1s and N1s spectra by X-ray photoelectron spectroscopy (XPS) verified N-substitution into TiO2 lattice which is responsible for the band gap narrowing. The oxygen total ratio (rOT) in the gas mixture had profound influence on the photocatalytic properties of the N-doped films. At a ratio of rOT≦0.18, heterogeneous phases, such as titanium oxynitride (TiNxOy) and titanium nitride (TiN), with low crystallinity were formed in the TiO2 lattice, showing low photocatalytic activity. On the other hand, the N-TiO2 films prepared at rOT≧0.20 were found to have improved photocatalytic activity on methylene blue degradation under visible-light irradiation at wavelengths longer than 500 nm, due to the substitutional N ions in the TiO2 lattice.