Although numerous papers are available about the origin of visible light photocatalytic activity of N-doped TiO 2 , it still remains conflicting how nitrogen-doping affects the visible light photocatalytic activity of TiO 2 . Thus N-doped TiO 2 was prepared by heat treatment of commercial P25-TiO 2 in flowing NH 3 , aiming at revealing the origin of visible light sensitization of N-doped TiO 2 . The resulting N-doped TiO 2 was characterized by means of electron spin resonance (ESR), X-ray photoelectron spectroscopy (XPS), diffusion reflectance spectrometry (DRS), and X-ray diffraction (XRD). Results indicate that N-doped TiO 2 shows triplet g value ESR signals (g=1.987, 2.004 and 2.024), which is attributed to single-electron-trapped oxygen vacancy (denoted as SETOV) in a certain chemical environment. Its visible light photocatalytic activity is proportional to the intensity of the triplet g value signals, which implies that the visible light photocatalytic activity of N-doped TiO 2 is closely correlated to the formation of SETOV during heat treatment in flowing NH 3 . Besides, N-doped TiO 2 catalyst calcinated at 600°C possesses the highest photocatalytic activity, but that calcinated at 700°C has drastically decreased photocatalytic activity and shows no XPS signal of nitrogen. Moreover, N-doped TiO 2 shows visible light absorption in a wavelength range of 400–520nm, which is attributed to the formation of SETOV and phase transformation from anatase to rutile. It is suggested that the visible light photocatalytic activity of N-doped TiO 2 is co-determined by the formation of SETOV in TiO 2 matrix and existence of doped-N on the surface. In other words, in the absence of either SETOV in TiO 2 matrix or doped-nitrogen on the surface, N-doped TiO 2 will not show visible light photocatalytic activity; and the higher the SETOV concentration is, the better the visible light photocatalytic activity will be.