In situ synthesis of mesoporous Co3O4 nanoparticles embedded in reduced graphene oxide nanosheets (GNSs) was implemented through coprecipitation method and pyrolytic process. In the charge-discharge process, electrolyte ions can fill with three-dimensional GNS matrix with connected pore network structures and mesoporous Co3O4 nanoparticles with uniform nanoscale structures as double ion buffer reservoirs. The capacitance value of Co3O4/GNS nanocomposites is 1085.6 F g(-1) at the current density of 2 A g(-1) in 2 M KOH solution. The composite has good cyclic stability, and the maximum capacitance loss is only 17.3% in 10,000 successive cycles.