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We have successfully produced bimetallic PtNi alloy nanoparticles on poly (diallyldimethylammonium chloride) (PDDA)-modified graphene nanosheets (PtNi/PDDA-G) by the "one-pot" hydrothermal method. The size of PtNi alloy nanoparticles is approximately 2-5 nm. The PDDA-modified graphene nanosheets (PDDA-G) provides an anchored site for metal precursors; hence, the PtNi nanoparticles could be easily bond on the PDDA-G substrate. PtNi alloy nanoparticles (2-5 nm) display a homogenous alloy phase embedded on the PDDA-G substrate, evaluated by Raman, X-ray diffractometer (XRD), thermal gravity analysis (TGA), electron surface chemical analysis (ESCA), and electron energy loss spectroscopy (EELS). The Pt/Ni ratio of PtNi alloy nanoparticles is ~1.7, examined by the energy dispersive spectroscopy (EDS) spectra of transmitting electron microscopy (EDS/TEM spectra) and mapping technique. The methanol electro-oxidation of PtNi/PDDA-G was evaluated by cyclic voltammetry (CV) in 0.5 M of H2 SO4 and 0.5 M of CH3 OH. Compared to Pt on carbon nanoparticles (Pt/C) and Pt on Graphene (Pt/G), the PtNi/PDDA-G exhibits the optimal electrochemical surface area (ECSA), methanol oxidation reaction (MOR) activity, and durability by chrono amperometry (CA) test, which can be a candidate for MOR in the electro-catalysis of direct methanol fuel cells (DMFC). [ABSTRACT FROM AUTHOR]