assembled Pt nanoparticle electrode and membrane-electrode-assembly (MEA) of polymer electrolyte fuel cells (PEFC) have been successfully prepared by using charged Pt nanoparticles. The charged Pt nanoparticles were prepared by alcoholic reduction in the presence of ionic poly(diallyldimethylammonium chloride) (PDDA) and diallyldimethylammonium chloride (DDA) stabilizers. A MEA with Pt loading of 2.8 ± 0.1 μ g cm(-2) was fabricated by the self-assembly of the charged Pt nanoparticles to the Nafion membrane surface probably via the sulfonic acid function sites, SO3-. The performance of the self-assembled MEA was 2.3 mW cm(-2), corresponding to a Pt utilization of 821 W per 1 g Pt. The Pt- DDA and Pt-PDDA nanoparticle showed significant electrochemical catalytic activity for the O-2 reduction reactions. The results show that the self-assembled Pt nanoparticles were able to form a Pt monolayer and such a monolayered structure could potentially offer a powerful tool in the fundamental studies in the PEFC systems. © 2005 The Electrochemical Society. All rights reserved.