In this paper, for improving the photocatalytic efficiency of titania (TiO₂) nanoparticles (NPs), AgAu alloy-TiO₂core-shell NPs are fabricated via a sol-gel (SG) process in the presence of AgAu alloy NPs with block copolymer shells as templates. The photocatalytic activities of the AgAu-TiO₂NPs on the photodecomposition of methylene blue (MB) are investigated. The AgAu-TiO₂composite NPs coated with 5.0% titania related to block copolymers show higher photocatalytic activity than the other samples in which the titania contents are larger than 5.0%. The results indicate that the increase of the thickness of the TiO2 shell leads to the decrease of the photocatalytic activity.