Pure TiO2, Sm3+ and Gd3+ co-doping TiO2 have been prepared by sol-gel method and characterized by the techniques such as XRD and SEM. The photocatalytic degradation of methylene blue (MB) in aqueous solution was used as a probe reaction to evaluate Pure TiO2, Sm3+ and Gd3+ co-doping TiO2 photocatalytic activity. The matrix distortion of TiO2 increases after co-doping of Sm3+ and Gd3+ are clearly observed. The results show that co-doping of Sm3+ and Gd3+ 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. When the co-doped amounts for Sm3+ and Gd3+ are 0.1% and 0.2%, its degradation rate reaches 99%.