A hybrid material composed of high density graphene/nickel oxide (RG/NiO) has been prepared by microwave-assisted in situ method of NiO at the defects of graphene. Graphene (RG) is obtained by thermal expansion of graphite oxide (GO). Graphene/nickel oxide composite is prepared by hydrothermal synthesis of aqueous mixture of RG and Ni(NO3)(2), followed by microwave irradiation of the hydrothermal synthesis product. SEM and TEM images show that NiO with size of 20-50 nm are uniformly distributed on the RG substrate. RG/NiO composite electrodes exhibit good electrochemical performance with a maximum specific capacitance of 617 F g(-1) at the charge/discharge current density of 1 A g(-1) with 5 M NaOH electrolyte. After 5000 cycling DC tests, a high level specific capacitance of 613 F g(-1) is obtained (99.4% retention of the initial capacity), suggesting excellent long-term cycling stability. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.