The nitrogen-doped photocatalyst TiO2-xNx is prepared by ball milling of TiO2 in a NH3 (.) H2O solution, and the photocatalyst is characterized by the UN-vis diffuse reflection spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD) and Transmission electron microscopy (TEM). Based on the results of the characterization, the mechanisms of the photocatalyst TiO2-xNx formation and the increase in the photocatalytic activity are investigated. The results show that, compared with TiO2, the photoabsorption of nitrogen-doped TiO2-xNx photocatalyst increases and red shifts of about 20 nm. The crystal phase composition of TiO2-xNx is the same as both TiO2 ball milled in H2O and TiO2 without ball milling. However, the diffraction intensity of the primary crystal faces of TiO2-xNx is comparatively weaker than that of TiO2 ball milled in H2O and TiO2 without ball milling. The crystallite size of ball milled TiO2 is not obviously changed. After ball milling for 120 h, the amount of doped nitrogen is 0.26 wt% in the TiO2-xNx. (c) 2005 Elsevier B.V. All rights reserved.