Poly(diallyldimethylammonium chloride) (PDDA)-functionalized graphene nanosheets (GNSs) were employed for direct methanol fuel cell applications. PDDA-functionalized GNSs (PDDA-GNSs) distributed positive charges on the surfaces of GNSs, favoring electrostatic self-assembly of negatively charged PtCl6 2− ions on the PDDA-GNSs. Subsequent reduction process was conducted using ethylene glycol, resulting in uniformly deposited Pt nanoparticles on PDDA-GNSs. Carbon black was functionalized by PDDA as well using a similar process, and the composite was denoted as PDDA-C. Experiments on Pt nanoparticles deposited on PDDA-C and GNSs were performed in the same manner as well. Pt electrocatalysts supported on the PDDA-GNSs exhibited higher electro-catalytic activity for the electro-oxidation of methanol compared with both Pt supported on the GNSs and PDDA-C. Therefore, PDDA-GNS can be an ideal support for Pt in direct methanol fuel cells.