Manganese dioxide-graphene nanosheet (MnO2/GNS) hybrid is used as an alternative cathode catalyst for oxygen reduction reaction in air-cathode microbial fuel cell (MFC). The results show that the nanostructured MnO2/GNS composite exhibit an excellent catalytic activity for oxygen reduction reaction due to MnO2 nanoparticles closely anchored on the excellent conductive graphene nanosheets. As a result, MFC with MnO2/GNS as air cathode catalyst generates a maximum power density of 2083 mW m(-2), which is higher than that of MFC with pure manganese dioxide catalyst (1470 mW m(-2)). Therefore, MnO2/GNS nanocomposite is an efficient and cost-effective cathode catalyst for practical MFC applications. (C) 2012 Elsevier B.V. All rights reserved.