Layered graphene/MnO2 composite was successfully synthesized by an electrostatic self-assembly process. First, positively charged colloidal MnO2 nanosheets and negatively charged colloidal graphene nanosheets were uniformly mixed; then graphene nanosheets were assembled on MnO2 nanosheets via electrostatic attraction. The structure and morphology of the obtained material were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results shown the mass ratio of graphene was about 4.4%. The capacitive properties of the samples were characterized by means of cyclic voltammetry (CV), galvanostatic charge discharge (CD), and electrochemical impedance spectrum (EIS) in 1 M Na2SO4 aqueous solution. The specific capacitance of graphene/MnO2 composite was found to be 280 F g(-1) at a current density of 0.2 A g(-1). Remarkably, its specific capacitance sharply increased during the initial 1000 cycles and then slowly increased to its maximum value at about the 3000th cycle, indicating that an electrochemical activation process occurred during the initial thousands of cycles. The mechanism of electrochemical activation is discussed in detail. Additionally, over 96.8% of maximum capacitance was retained after 10,000 cycles, exhibiting its excellent cycle stability. The enhanced specific capacitance and excellent cycle performance of graphene/MnO2 composite compared to pure MnO2 were attributed to the synergic effect of highly specific capacitance of MnO2 and excellent conductivity of graphene. Our experimental results demonstrated that graphene/MnO2 composite is a promising candidate as an electrode material for supercapacitors. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.