Novel hierarchical composites of reduced graphene oxide/acid-treated multi-walled carbon nanotubes (RGO/aMWCNT) are prepared by Co-60 gamma-ray irradiation in isopropanol/water under an inert atmosphere. The irradiation reduction mechanism of graphene oxide is further investigated and verified. The prepared RGO/aMWCNT composites display a three-dimensional interpenetrating network structure. And the composite with the GO/aMWCNT weight ratio of 80/20 possesses high conductivity (1820 S m(-1)), large specific surface area (1420 m(2) g(-1)) and low oxygen content. As a result, it shows improved electrochemical capacitance (227 F g(-1)), excellent cycling stability (4.9% capacitance loss after 10,000 cycles) and rate capability. Furthermore, this irradiation reduction technique used to prepare the RGO/aMWCNT composites is readily scalable to industrial levels. (C) 2013 The Authors. Published by Elsevier B.V. All rights reserved.