By using KMnO4 as both the oxidant and Mn source in a modified Hummers’ method, a facile and economical procedure is proposed to synthesize graphene oxide/MnO2 composite. Then after annealing at 400°C, the graphene/Mn3O4 composite with 70.7wt.% Mn3O4 nanoparticles homogeneously anchored on each side of graphene nanosheets is obtained. The electrochemical performance of the graphene/Mn3O4 is investigated as a supercapacitor electrode material. It exhibits the high specific capacitance of 271.5Fg−1 at the current density of 0.1Ag−1 as well as great cycle performance. At the high current density of 10Ag−1, the capacitance retention even keeps about 100% after 20,000 cycles. The good electrochemical performance and the simple accessibility make this graphene/Mn3O4 composite a promising candidate as a supercapacitor electrode material.