MnOx nanocrystalline/graphene hybrid electrodes are prepared by exchanging graphene oxide (GO) with Mn2+ ions and followed by calcining the Mn2+-exchanged material (Mn2+-GO) at different temperatures in N-2 atmosphere. The effect of the calcining temperatures on the phase and morphology of the obtained materials is investigated. The phase and size of MnOx nanocrystalline and the reduction degree of GO in MnOx nanocrystalline/graphene hybrids are affected by calcining temperature. Mn3O4/RGO-450 hybrid electrode with a little of wrinkle degree of the reduced graphene oxide (RGO) is obtained when sample Mn2+-GO was calcined at 450 degrees C, in which Mn3O4 nanocrystalline with average size of 13.5 nm are homogeneously distributed on the surface of RGO. The electrochemical properties of MnOx nanocrystalline/graphene hybrid electrodes are investigated by cyclic voltammetry and galvanostatic charge-discharge in 1 M KOH electrolyte. Their electrochemical performance is highly structure dependent and the highly utilization of MnOx nanocrystalline. The Mn3O4/RGO-450 hybrid electrode exhibits a high specific capacitance of 517 F g(-1) at a current density of 1 A g(-1). This method can be expanded to control the size of other transition-metal oxide nanocrystalline on the graphene nanosheets and improve the capacitance and utilization of the metal oxide-graphene nanohybrids. (C) 2015 Elsevier Ltd. All rights reserved.