This paper reports a facile one-step hydrothermal treatment of graphene oxide (GO) and cobalt acetate (Co(Ac)(2)) for preparing reduced GO (rGO)/Co3O4 composites which were used as electrode materials for supercapacitors containing electrolytes of 2 M KOH aqueous solution. The morphologies and structures of rGO/Co3O4 composites were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Raman spectrum, and N-2 adsorption-desorption isotherms. The electrochemical performances of two-electrode supercapacitors were evaluated by cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy techniques. During the hydrothermal reaction, GO was reduced and 10-30 nm-sized Co3O4 nanoparticles were in situ grown onto the rGO sheets simultaneously. The effects of mass ratios of GO and Co(Ac)(2) on the performances of supercapacitors were investigated. In comparison with pure Co3O4-based supercapacitor, supercapacitors based on rGO/Co3O4 composites show better performances because both the specific surface areas and the electrical conductivities of electrode materials were increased by the introduction of rGO. When the mass ratio of GO and Co(Ac)(2) is 1:2, rGO/Co3O4 composite electrode exhibits the highest capacitance of 263.0 F/g at a constant current density of 0.2 A/g in a two-electrode supercapacitor. In addition, the supercapacitor shows high rate capability and long cyclic durability.