The iridium oxide nanoparticles supported on Vulcan XC-72 porous carbon were prepared for cathode catalyst in polymer electrolyte fuel cell (PEFC). The catalyst has been characterized by transmission electron microscopy (TEM) and in PEFC tests. The iridium oxide nanoparticles, which were uniformly dispersed on carbon surface, were 2-3 nm in diameter. With respect to the oxygen reduction reaction (ORR) activity was also studied by cyclic voltammetry (CV), revealing an onset potential of about 0.6V vs. an AgiAgCl electrode. The ORR catalytic activity of this catalyst was also tested in a hydrogen-oxygen single PEFC and a power density of 20 mW cm(-2) has been achieved at the current density of 68.5 mA cm(-2). This study concludes that carbon-supported iridium oxide nanoparticles have potential to be used as cathode catalyst in PEFC. Crown Copyright (C) 2010 Published by Elsevier B.V. All rights reserved.