Gold nanoparticles (Au NPs) were directly deposited on commercially available poly(methyl methacrylate) (PMMA) beads by the deposition reduction method by adding dropwise of NaBH 4 into the aqueous solution of Au(en) 2 Cl 3 . The average diameter of Au NPs was 6.9nm and, in contrast to the case of rapid addition, there were no aggregated particles larger than 20nm. Catalytic activity of Au/PMMA was tested for the reduction of 4-nitrophenol to 4-aminophenol with an excess amount of NaBH 4 . Gold NPs on PMMA showed the highest catalytic activity among polymer supported Au NPs previously reported. The reaction rate is the pseudo-first-order with respect to 4-nitrophenol. The pseudo-first-order rate constant and the activation energy were estimated to be 7.2–7.9×10 −3 s −1 at 295K and 38kJmol −1 , respectively.