Platinum nanoparticles were successfully prepared in an aqueous solution using a perfluorinated sulfonic acid copolymer (PFSA) as a protecting agent. PFSA having good proton conductivity, gas permeability and chemical stability is used as a polyelectrolyte and an ionomer in catalyst layers of polymer electrolyte fuel cells. It was confirmed by infrared spectroscopy and in situ atomic force microscopy that the Surface of nanoparticles was covered with PFSA molecules. In addition, PFSA-protected platinum nanoparticles formed the nano-network on dried films. Porous structures might improve the diffusion of reactants and products in the catalyst layer. The PFSA-protected platinum nanoparticles showed as good electrocatalytic activity for oxygen reduction reaction as a conventional Pt/C catalyst. (C) 2009 Elsevier B.V. All rights reserved.