The clew-like MnO2, synthesized by a low-temperature liquid-phase reaction, were thermally treated (300-700 degrees C) under nitrogen atmosphere to improve its capacitive properties. Crystal structure, porosity, conductivity, microstructure and the electrochemistry properties of thermal treated under nitrogen MnO2 electrode were measured. The result indicates that thermal treatment under nitrogen can improve MnO2 electrode's capacitive properties and the maximum specific capacitance is 368.2 F/g at 1 A/g in a 6 mol/L KOH electrolyte for the prepared electrode annealed at 400 degrees C, which was increased by 102.4% compared with untreated sample (181.9 F/g). Further observation of MnO2 electrode's microstructure showed that oxygen deficiency -induced phase transformation during thermal treatment under nitrogen leads to co-existence of multivalent Mn, which could improve its electrical conductivity and surface properties, then enhancing its electrochemical performance. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.