Fe3O4/reduced graphene oxide (rGO) composites were prepared by a two-step method: solution precipitation followed by hydrogen reducing. The mass ratio of Fe3O4 to rGO in the composites could be carefully controlled in the first step. The structure and morphology of Fe3O4/rGO composites were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy and Raman spectroscopy. The electrochemical properties of the composites as electrode of supercapacitors were investigated using cyclic voltammetry and galvanostatic charge–discharge tests. It was found that the mass ratio of Fe3O4 to rGO had great effects on the crystal size of Fe3O4, specific surface areas and pore volume, as well as the specific capacitance of the composites. The capacitance of the composite was higher than that of any individual component due to the synergetic effect between Fe3O4 and rGO. The composite exhibited an excellent cyclic stability and could maintain the initial capacity over 1000 cycles without any decay. This report provides useful clues in design and synthesis graphene-based composites for the applications as energy materials.