Ruthenium oxide/graphene (RuO2/GR) hybrid materials for high performance electrochemical capacitor have been prepared by a solution-phase assembly technology between RuO2 nanosheets and GR nanosheets at room temperature. The high dispersion of RuO2 and GR nanosheets maintains a high structural stability for the hybrid material, and causes an obvious synergistic effect between the RuO2 and GR nanosheets. A specific capacitance of 479 F g−1 has been obtained for the hybrid material with RuO2 mass content of 40% (abbreviated as RuGR46), and a high specific capacitance of 998 F g−1 obtained for RuO2 in the electrode. The utilization of RuO2 in the RuGR46 hybrid material increases by adding GR, and the capacitance of RuGR46 is quite comparable to that of the pristine RuO2·xH2O while 60 wt% of RuO2 can be saved. A symmetrical electrochemical capacitor based on the RuGR46 electrode is assembled with 0.5 mol L−1 H2SO4 solution as the electrolyte in a voltage of 0–1.2 V. It can give a high energy density of 20.28 Wh kg−1 at a power density of 600 W kg−1. Moreover, it presents a high power density (14.03 Wh kg−1 at 12 kW kg−1) and excellent cycle performance.