Activated carbon nanospheres (ACNS) with hierarchical porosity have been synthesized via a low-concentration hydrothermal reaction with phenolic resin as a carbon source and block copolymer F127 as a soft template, followed by subsequent KOH activation. As-obtained ACNS material integrates several advantages, such as a nanoscale particle size, a hierarchical porosity, abundant oxygen-containing functional groups and a high mass density (0.81 g cm(-3)). The electrochemical tests in KOH aqueous electrolyte shows superior capacitive performances, i.e. both high gravimetric and volumetric capacitance of 243 F g(-1) and 198 F cm(-3), a large areal capacitance of 25.5 mu F cm(-2), an excellent rate capability as well as a superior long-term cycling stability (96.1% capacitance retention over 10000 cycles). Moreover, symmetric supercapacitors based on ACNS electrodes achieve a high volumetric energy density of 6.0 Wh L-1, which maintains 3.7 Wh L-1 even at a high power density of 6.6 kW L-1. Such ACNS thus shows great potential as a promising porous carbon material for high volumetric performance supercapacitors. (C) 2015 Elsevier Ltd. All rights reserved.