The carbon monoxide (CO) tolerance performance of polymer electrode fuel cells (PEFCs) was studied for a catalyst composed of graphene nanosheets (GNS) with sub-nano-Pt clusters. The Pt catalysts supported on the GNS showed a higher CO tolerance performance in the hydrogen oxidation reaction (HOR), which was significantly different from that of platinum on carbon black (Pt/CB). It is proposed that the presence of the sub-nano-Pt clusters promotes the catalytic activity and that the substrate carbon material alters the catalytic properties of Pt via the interface interactions between the graphene and the Pt.