A novel catalyst support constructed from poly (diallydimethylammonium chloride) functionalized graphene (PDDA-GN) and H3PMo12O40 (PMo12) was prepared by layer by layer (LBL) electrostatic assembly and were used for electro-deposition of Pd particles in situ. The Pd/{PDDA-GNs/PMo12}(n) composite films were characterized by Raman, XPS, EDS, XRD, AFM and TEM. The composite films presented good electrocatalytic activity, better electrochemical stability and improved tolerance of CO in cyclic voltammetry, chronoamperometry and CO-stripping voltammetry tests. Especially, the catalytic activity of the as prepared composite toward formic acid electro-oxidation is 4.1 times of the commercial Pd/C catalyst. It indicates that PDDA-GNs preserve superior electric conductivity of graphene sheets in the composite films, and the introduction of electroactive PMo12 into the composite films contributes to convert intermediate species CO into CO2. The synergistic effects of PDDA-GNs and PMo12 remarkably enhance the electrocatalytic activity and stability of nano-Pd regarding formic acid oxidation. (C) 2014 The Electrochemical Society. All rights reserved.