In this study, the formation of silver nanoparticles (Ag NPs) with a particle size of about 3 nm was successfully achieved by using the mesoporous channels of carboxylic acid (-COOH) functionalized SBA-15 as the support. When pure silica SBA-15 (without -COOH groups) was employed, the Ag NPs were formed outside of the mesopore and their particle size was significantly larger, up to about 20 nm. The -COO- groups under basic conditions can effectively interact with the Ag+ ions, and thus allowed facile fabrication of Ag NPs. The catalytic activity of these Ag NPs based SBA-15 materials were tested for a model reaction, namely the reduction of 4-nitrophenol to 4-aminophenol. The results showed that the particle size of the Ag NPs play a key role in determining their catalytic activity. The apparent kinetic constant for the Ag NPs with a particle size of about 3 nm was 1.1 × 10-2 s-1, corresponding to the activity parameters of 12.2 and 4630 s-1 g-1 by considering the total mass of the catalyst used and the Ag NPs alone, respectively, which were remarkably high as compared to other matrices bearing Ag. Moreover, the Ag NP based materials exhibited good recyclability up to 5 cycles.