Ag clusters (mean diameter = 1.5 nm, standard deviation = 0.37 nm) were photodeposited on TiO2 particles in a highly dispersed state. The loading of a small amount of the Ag clusters (0.24 wt %) dramatically enhanced both the activity for the TiO2 photocatalytic reduction of nitrobenzene and the product selectivity of aniline. The essential action mechanism of the Ag clusters is discussed.