Graphene nanosheets (GNSs) with narrow mesopore distribution around 4nm were mass-produced from natural graphite via the oxidation and rapid heating processes. The effects of oxidant addition on the morphology, structure and electrochemical performance of GNSs as electrode materials for electric double-layer capacitor (EDLC) were systematically investigated. The electrochemical properties of EDLC were influenced by the specific surface area, pore characteristics, layer stacking and oxygen-containing functional group contents of electrode materials. Deeper oxidation makes graphite possess both higher specific surface area and more graphene edges, which are favorable for the enhancement of capacitive performance of EDLC. The electrodes with freestanding graphene nanosheets prepared by coating method exhibited good rate capability and reversibility at high scan rates (to 250mVs−1) in electrochemical performances. GNS electrode with specific surface area of 524m2 g−1 maintained a stable specific capacitance of 150Fg−1 under specific current of 0.1Ag−1 for 500 cycles of charge/discharge.