Graphene/multiwall carbon nanotubes/MnO2 (GR/MCNTs/MnO2) hybrid material with a specific capacitance of 126Fg−1 within a potential window of 0–1.1V vs. saturated calomel electrode has been synthesized by a simple redox reaction between graphene/multiwall carbon nanotubes (GR/MCNTs) and KMnO4 at room temperature. The morphology and structure of the obtained material are examined by XRD, SEM and TEM. The electrochemical properties are characterized by cyclic voltammetry, galvanostatic charge–discharge and electrochemical impedance spectroscopy. The mass percentage of MnO2 with layered structure is 37% in the hybrid material. An asymmetrical electrochemical capacitor (EC) is assembled using GR/MCNT/MnO2 hybrid material as positive electrode and GR/MCNT material as negative electrode, respectively. The electrochemical properties of the two electrodes and the asymmetrical EC are investigated in 1molL−1 Na2SO4 aqueous electrolyte. The asymmetrical EC can cycle reversibly in a cell potential of 0–2.0V and gives a high energy density of 28.33Whkg−1, which is much higher than those of symmetrical ECs based on GR/MCNT/MnO2 (6.20Whkg−1) and GR/MCNT (3.92Whkg−1). Moreover, the asymmetrical EC presents a high power density (5kWkg−1 at 13.33Whkg−1) and excellent cycling performance of 83% retention after 2500 cycles.