A series of pure and Y3+-doped TiO2 nanoparticles with high photocatalytic activities were prepared by a sol-gel method using tetra-n-butyl titanate as precursor. The as-prepared catalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and diffuse reflectance spectroscopy (DRS). The results indicated that yttrium doping could effectively reduce the crystalline size, inhibit the anatase-to-tutile phase transformation and surppress the recombination of the photogenerated electron-hole pairs. The DRS results showed that the optical absorption edge shifted to red direction owing to yttrium ion doping. The photocatalytic activities of samples were evaluated by the photodegradation of methyl orange (MO) aqueous solution under 300 W high pressure mercury lamp irradiation. Photodegradation results revealed that Y3+ doping could greatly improve the photocatalytic activity of TiO2. In this experiment, the optimal dosage was 1 .5 mol.% when samples were calcined at 773 K for 2 h, which caused a MO photodegradation rate of 99.8% under UV irradiation for 70 min.