Co3O4–reduced graphene oxide (rGO) nanocomposites are prepared by co-precipitation of Co(OH)2 with graphite oxide (GO) to form a Co(OH)2–GO precursor, followed by thermal treatment. It is found that the ratio between Co3O4 and rGO has a significant effect on their electrochemical activities. The specific capacitance of 636 F g−1 is achieved when the mass ratio of Co3O4 to rGO is equal to 80.3:19.7. A novel asymmetric supercapacitor is further fabricated with the Co3O4–rGO nanocomposite as the anode and activated carbon as the cathode in 6 M aqueous KOH solution as electrolyte. The assembled asymmetric supercapacitor can cycle reversibly in a voltage of 0–1.5 V and exhibits a high energy density of 35.7 Wh kg−1 at power density of 225 W kg−1. Moreover, the asymmetric supercapacitor shows an excellent cycling stability with capacitance retention of 95% after 1000 cycles at a current density of 0.625 A g−1.