Platinum hollow nanospheres (HNPt) are synthesized via a galvanic replacement reaction between as-prepared cobalt nanoparticles and Pt4+ (H2PtCl6) ions. Support graphene nanosheets are also prepared using a chemical route for HNPt immobilization. Morphology and chemical composition of the as-prepared nanocomposite are characterized by transmission electron microscopy and energy dispersion X-ray spectroscopy, respectively. The prepared nanocomposite electrocatalytic activity toward sodium borohydride oxidation is studied by electrochemical techniques. To investigate the morphology effect of electrocatalytic behavior, Pt solid nanoparticles are also synthesized and immobilized on graphene support (NPt/G), and its electrocatalytic activity is studied. Results show that the electrocatalyst's morphology has a significant effect on electrocatalytic activity, as HNPt/G shows a superior electrocatalytic activity in comparison with NPt/G toward sodium borohydride oxidation.