A series of cobalt oxide/graphene nanocomposites have been successfully synthesized by adding carbon blacks filler. We have introduced carbon blacks to act as a structural modifier in fabricating graphene based composites. The composites were prepared by ultrasonication and microwave-assisted methods. Microstructure measurements showed that carbon blacks could be conductive links among layers of graphene and cobalt oxide particles (5–7nm in size), which was deposited on the edge surfaces of nanosheets. This composite could function as a stable structured blocks for preventing an aggolomeration of graphene layers. The electrochemical properties are investigated by cyclic voltammetry (CV), galvanostatic charge/discharge and electrochemical impedance spectroscopy (EIS). The prepared nanocomposites showed the superior capacitive performance with good rate capability, large specific capacitance, and excellent cyclic performance. Among various samples, Co3O4/GNS–CB (15wt.%) showed the largest specific capacitance of 341Fg−1 at a scan rate of 10mVs−1 in 6M KOH electrolyte. Therefore, the prepared composite could be potential electrode materials for supercapacitors.