Birnessite-type MnO2 nanoparticles have been successfully synthesized by the microwave-assisted reflux as short as 5 min. The microstructure and morphology of the products were characterized by X-ray diffraction, N-2 adsorption-desorption isotherms, scanning electron microscopy, transmission electron microscopy. The electrochemical properties of the as-prepared MnO2 as an electrode material for supercapacitor were investigated by cyclic voltammetry and galvanostatic charge-discharge measurements in 1 M Na2SO4 electrolyte, and a high specific capacitance of 329 Fg(-1) was achieved at a current density of 0.2 Ag-1. The specific capacitance retention was 92% after 1000 cycles at 2 Ag-1, suggesting that it is a promising electrode material for supercapacitors. (C) 2015 Elsevier Ltd. All rights reserved.