The un-doping, single-doping and co-doping TiO2 nanoparticles have been prepared by sol-gel method with Ti(OC4H9)(4) as a raw material and characterized by X-ray diffraction (XRD) and UV-vis absorption spectra, and their photocatalytic activities have been investigated by photocatalytic oxidation of methyl orange. It is found that Fe3+-doping broadens the absorption profile, improves photoutilization of TiO2, and then generates more electron-hole pairs. Ho3+-doping restrains the increase of grain size, leads to crystal expansion and matrix distortion and retards the recombination of the photoexcited charge carriers. The photocatalytic activity of TiO2 co-doped with Fe3+ and Ho3+ ions is markedly improved due to the cooperative actions of the two dopants. (C) 2007 Elsevier B.V. All rights reserved.