Electrode properties of defect-introduced graphene sheets (GSs) in an acid aqueous electrolyte as an electrode material for electrochemical capacitors were investigated. Defects were successfully introduced into graphene structures through repeated oxidation of graphite followed by rapid thermal exfoliation. The specific surface area was increased up to 991 m(2) g(-1) by the defect introduction process. A GS electrode with a high amount of defects maintained a stable specific capacitance of 136 F g(-1) at a current density of 500 mA g(-1) for 1000 cycles, which is larger than that of GSs with a low amount of defects. A large specific capacitance of 115 F g(-1) was also exhibited at a high current density of 5000 mAg(-1). These capacitances were almost maintained even after the specific surface area was reduced by the sheet-restacking process, indicating that the large capacitance is derived from the defects introduced in the graphene structure. (C) 2014 Elsevier Ltd. All rights reserved.