Polyaniline (PAn) sensitized nanocrystalline TiO 2 composites (TiO 2 /PAn) were successfully prepared and used as an efficient photocatalyst for the degradation of dye methylene blue (MB). The results showed that PAn was able to sensitize TiO 2 efficiently and the composite photocatalyst could be activated by absorbing both the ultraviolet and visible light (λ: 190∼800nm), whereas pure TiO 2 absorbed ultraviolet light only (λ<380nm). Under the irradiation of natural light, MB could be degraded more efficiently on the TiO 2 /PAn composites than on the TiO 2. Furthermore, it could be easily separated from the solution by simple sedimentation.