Pure TiO2, Eu3+ and Sm3+ co-doping TiO2 composite nanoparticles have been prepared by sol-gel method and characterized by the techniques such as XRD, SEM and DRS. The photocatalytic degradation of methylene blue (MB) in aqueous solution was used as a probe reaction to evaluate their photocatalytic activity. The matrix distortion of TiO2 nano-particles increases after co-doping of Eu3+ and Sm3+ and a blue-shift of the absorption profile are clearly observed. The results show that co-doping of Eu3+ and Sm3+ inhibits the phase transformation of TiO2 from anatase to rutile, decreases the diameter of TiO2 nano-particles and significantly enhance the photocatalytic activity of TiO2. The Eu3+ and Sm3+ co-doped into TiO2 nano-particles exert a synergistic effect on their photocatalytic activity.