A novel ball-milled graphite/Mn3O4 composite has been prepared directly through the reaction between high-energy ball-milled graphite and potassium permanganate under microwave irradiation. The microstructure and morphology of the composite have been investigated by X-ray diffraction and transmission electron microscopy. Three-electrode investigations show that the ball-milled graphite/Mn3O4 composite has a high potential window (ΔE =1.9V) in 1M Na2SO4 aqueous solution. A symmetric electrochemical capacitor has been built using graphite/Mn3O4 composite with 1M Na2SO4. This system exhibits ideal supercapacitive properties and stable cycle behavior while the cell voltage is set to as high as 1.8V.