In this work, γ-MnO2 nanoparticles and α-MnO2 urchin-like nanostructures have been successfully synthesized by the microwave-assisted reflux as short as 5min 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 N2 adsorption–desorption. γ-MnO2 nanoparticles showed a smaller particle size, a higher specific surface area, and a larger pore volume than those of α-MnO2 urchin-like nanostructures. The electrochemical performance was investigated by using cyclic voltammetry and galvanostatic charge–discharge techniques. γ-MnO2 nanoparticles obtained from the neutral condition exhibited a capacitance of 311Fg−1 at a current density of 0.2Ag−1, which is much higher than that of α-MnO2 urchin-like nanostructures (163Fg−1). The specific capacitance retention and coulombic efficiency after 5000 cycles at 1Ag−1 were about 93% and almost 100% for γ-MnO2 nanoparticles, respectively, suggesting that it is a promising electrode material for supercapacitors.