N-Doped TiO2 photocatalysts were prepared by a hydrothermal method with tetra-n-butyl titanate (TTNB) and triethanolamine as precursors. The obtained samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and UV-visible diffuse reflectance spectra (DRS), respectively. Photocatalytic activities of the anatase products were investigated on the degradation of methyl orange (MO). The incorporation of nitrogen impurity in anatase TiO2, was studied by the first-principles calculations based on the density functional theory (DFT). The calculated electronic band structures for substitutional and interstitial N-doped TiO2, indicated the formation of localized states in the band gap, which lied above the valence band. Excitation from the impurity states of N 2p to the conduction band could account for the optical absorption edge shift toward the lower energies. It was consistent with the experimentally observed absorption of N-doped samples in the visible region.