A visible light responsive N, Mo co-doped $TiO_2$ were prepared by sol-gel method. X-ray diffraction, TEM, $N_2$ adsorption, UV-vis spectroscopy, photoluminescence, and X-ray photoelectron spectroscopy were used to characterize the prepared $TiO_2$ samples. Doping restrained the phase transformation from anatase to rutile and reduced the particle sizes. The band gap was much narrowed after N, Mo co-doping. The photocatalytic activities were tested in the degradation of an aqueous solution of a reactive dyestuff, methylene blue, under visible light. The photocatalytic activities of doped $TiO_2$ were much higher than that of neat $TiO_2$. The photocatalytic stability of N, Mo co-doped $TiO_2$ was much better than that of N doped $TiO_2$.