Abstract. Pure TiO 2 and N-doped nano-TiO 2 photo catalysts were individually prepared following the sol-gel method under ultrasonic conditions. The photo catalytic decomposition of methyl orange (MO) solution under ultraviolet and visible light irradiation was used as a probe reaction to estimate their photo catalytic activities. The particle size, crystal structure, and optical properties of the prepared TiO 2 were performed by X-ray diffraction, transmission electron microscopy and UV-vis diffuse reflectance spectroscopy. The results suggest that ultrasound significantly reduced the size of the TiO 2 particles, improved particle dispersion, which in turn improve the photo catalytic activity of TiO 2 under ultraviolet light. Ultrasound also promoted the doping of non-metal nitrogen and markedly enhanced the visible light absorption capacity of the N-doped nano-TiO 2 . Compared with pure TiO 2 , the degradation rate for MO under visible light of U-N-TiO 2 was increased by 70%.