Nitrogen-doped TiO2 (N-TiO2) nanoparticles have been successfully prepared via a direct and simple hydrothermal reaction of a commercial Degussa P25 with triethanol amine as solvent and nitrogen source. As-prepared N-TiO2 was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), ultraviolet–visible light (UV–vis) absorption spectra, electron probe microanalysis (EPMA) and X-ray photoelectron spectroscopy (XPS) techniques. The results confirm that hydrothermal reaction is an effective way to incorporate nitrogen into the TiO2 lattice, especially nitrogen substitute for titanium. The nitrogen concentration in TiO2 can be as high as 21% (molar ratio), which is described as Ti1-yO2-xNx+y (in this paper,x=0.36, y=0.27, i.e., Ti0.73O1.64N0.63). The chemical statuses of N have been assigned to N-Ti-O and O-N-O in the TiO2 lattice as identified by XPS. Photocatalytic degradation of methyl orange has been carried out in both UV-vis (simulated solar light) and the visible region (λ>4400 nm). N-TiO2 exhibits higher activity than the Degussa P25 TiO2 photocatalyst, particularly under visible-light irradiation. This study has developed a promising and practical pathway to new nitrogen-doped photocatalysts.