The new electrode materials with large specific capacitance, cycling stability and high rate capability are important to advance the development of supercapacitors. A nanocomposite of mesoporous MnO2 nanospheres anchered reduced graphene oxide (MG) was synthesized by a simple and inexpensive reflux reaction. KMnO4 refluxed with oxalic acid as reducing agent was used to the growth of mesoporous MnO2 nanospheres on graphene oxide (GO). The MG as an electrode exhibits a specific capacitance of 466.7Fg−1 at a current density of 1Ag−1, which is 3.33 times larger than that of pure MnO2 (140Fg−1) and 3.19 times of graphene (146Fg−1). The rate capability arrives at 10Ag−1 with specific capacitance of 454.8Fg−1. The capacitance retention is 92% over 2000 cycles at 1Ag−1. The electrochemical impedance spectroscopy (EIS) demonstrates roles of mesopores and RGO in facilitating electronic transport and ionic diffusion across the pores and interfaces.