Different valences of metal ions such as Fe3+ and Nb5+ were co-doped with nitrogen ion into titanium dioxide by hydrothermal method using metal chlorides and hexamethylenetetramine as the sources of metal ions and nitrogen ion, respectively. The co-doping of low-content metal ion showed no noticeable influence on the crystalline phases and specific surface area (S.S.A.) of the samples. Doping with Fe ion could significantly enhance the absorption in visible light region, but doping with Nb ion showed almost no effect. The photocatalytic activities of the samples were determined for the oxidative destruction of NO gas under various wavelengths. Co-doping with Nb ion improved the deNOx ability, but co-doping with Fe ion depressed it, indicating that co-doping with higher valence metal ion was effective in reducing the vacancy in the lattice which acts as the recombination center of the photo-induced electrons and holes, and achieving higher photocatalytic activity.