Absorption spectra of anatase and rutile TiO₂ modified with various sizes of Au nanospheres (5–60 nm) in the 150–300 nm region were measured by using attenuated total reflection spectroscopy. The smaller Au nanospheres induced larger spectral changes, which mean larger electronic state changes and higher charge-separation efficiency enhancements. In fact, TiO₂ with the smaller Au nanospheres showed the higher photocatalytic activities. In contrast, although Au nanorods with various aspect ratios or Au nanocubes were deposited instead of Au nanospheres on TiO₂, their spectra (i.e. electronic states) were not significantly changed. Therefore, it was revealed that while there was little shape dependence, the smaller Au nanoparticles induced the larger electronic state change. This may be due to the difference in the potential gradient generated in the interfacial region between TiO₂ and the metal, and the smaller Au nanoparticle deposition leads to the higher photocatalytic activities.