Asymmetric supercapacitor cells based on MnO2 as positive electrode and graphene as negative electrode have been constructed with 1M Na2SO4 aqueous electrolyte. Amorphous MnO2 nanoparticles were prepared by reducing KMnO4 in N,N-dimethylformamid (DMF). Graphene was prepared by hydrothermal reduction of exfoliated graphite oxide sheets. The MnO2/graphene asymmetric supercapacitor could operate reversibly at a high cell voltage of 2.0V, showing an energy density of 25.2Whkg−1 at a power density of 100Wkg−1, which is much higher than those of symmetric supercapacitors based on MnO2/MnO2 (4.9Whkg−1) and graphene/graphene (3.6Whkg−1). Moreover, the asymmetric system exhibits capacitance retention of 96% after 500 cycles.