In this work, gamma-MnO2 nanoparticles and alpha-MnO2 urchin-like nanostructures have been successfully synthesized by the microwave-assisted reflux as short as 5 min under neutral and acidic conditions, respectively. The crystal structure, morphology and chemical composition of the MnO2 products were investigated be X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and N-2 adsorption-desorption. gamma-MnO2 nanoparticles showed a smaller particle size, a higher specific surface area, and a larger pore volume than those of alpha-MnO2 urchin-like nanostructures. The electrochemical performance was investigated by using cyclic voltammetry and galvanostatic charge-discharge techniques. gamma-MnO2 nanoparticles obtained from the neutral condition exhibited a capacitance of 311 F g(-1) at a current density of 0.2 A g(-1), which is much higher than that of alpha-MnO2 urchin-like nanostructures (163 F g(-1)). The specific capacitance retention and coulombic efficiency after 5000 cycles at I A g(-1) were about 93% and almost 100% for gamma-MnO2 nanoparticles, respectively, suggesting that it is a promising electrode material for supercapacitors. (C) 2012 Elsevier Ltd. All rights reserved.