Polyaniline (PAn) sensitized nanocrystalline TiO2 composite photocatalyst (PAn/TiO2) with high activity and easy separation was facilely prepared by in situ chemical oxidation of aniline from the surfaces of the TiO2 nanoparticles. The morphology, structure, and light absorption properties of composite photocatalyst were examined in term of its application to photocatalysis. The photocatalytic activity of PAn/TiO2 nanocomposites for the degradation of methylene blue (MB) aqueous solution was investigated and compared with pure TiO2. The spectra analyses illustrated that, when PAn deposited on the surface of TiO2, the crystalline behavior of PAn was hampered and the degree of crystallinity decreased, and the characteristic peaks of the PAn were shifted indicating that there was a strong interaction between PAn and TiO2 nanoparticles. PAn was able to sensitize TiO2 efficiently and the composite photocatalyst could be activated by absorbing both the ultraviolet and visible light (lambda = 190-800 nm), whereas pure TiO2 absorbed ultraviolet light only (lambda < 400 nm). Photocatalytic experiments showed that under natural light irradiation, MB could be degraded more efficiently on the PAn/TiO2 than on the pure TiO2, due to the charge transfer from PAn to TiO2 and efficient separation of e(-)-h(+) pairs on the interface of PAn and TiO2 in the excited state. More significantly, the PAn/TiO2 composite photocatalyst exhibited easy separation and less deactivation after several runs. The advantages of the obtained PAn/TiO2 composite photocatalyst revealed its great practical potential in wastewater treatment.