Carbon-doped TiO2 nanoparticles were prepared by sol-gel auto-combustion method and characterized by X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), Brunauer-Emmett-Teller method (BET), UV-vis diffuses reflectance spectroscopy (DRS). UV-vis diffuse reflectance spectra showed that carbon-doped TiO2 exhibited obvious absorption in the visible light range. The visible light photocatalytic activity of carbon-doped TiO2 was ascribed to the presence of oxygen vacancy state between the valence and the conduction bands because of the formation of Ti3+ species in the as-synthesized carbon-doped TiO2. The sample calcined at 873 K showed the highest photocatalytic activity under solar irradiation. The effects of photocatalyst concentration, initial concentration of methylene blue, and pH value in aqueous solution were also presented. (C) 2008 Elsevier Ltd. All rights reserved.