NH3 plasma, N-2 plasma, and annealing in flowing NH3 were used to prepare N doped TiO2, respectively. XRD, UV-vis spectroscopy, N-2 adsorption, FT-IR, Zeta-potential measurement, and XP spectra were used to characterize the prepared TiO2 samples. The nitridation procedure did not change the phase composition and particle sizes of TiO2 samples, but extended its absorption edges to the visible light region. The photocatalytic activities were tested in the degradation of an aqueous solution of a reactive dyestuff, methylene blue, under visible light. The photocatalytic activity and stability of TiO2 prepared by NH3 plasma were much higher than that of samples prepared by other nitridation procedures. The visible light activity of the prepared N doped TiO2 was improved by increasing the lattice-nitrogen content and decreasing adsorbed NH3 on catalyst surface. The lattice-nitrogen stability of N-doped TiO2 samples improved after HCl solution washing. The possible mechanism for the photocatalysis was proposed. (C) 2011 Elsevier B.V. All rights reserved.