Ultrathin nanorods self-assembled porous nanoflowers to form hierarchical mesoporous beta-manganese dioxide (beta-MnO2) with extremely high specific surface areas of > 400 m(2) g(-1) are fabricated in high yield at room temperature in a simple 1,2-butanediol/KMnO4 system. As supercapacitor electrodes, specific capacitances of as-prepared beta-MnO2 nanoflowers can reach 500-430 F g(-1) at a current density of 0.1 A g(-1) in 0.5M Na2SO4 aqueous electrolytes. And greatly enhanced rate capability of 66.4% at 20 A g(-1) with a good cycling stability of 95.5% (10 A g(-1), after 5000 cycles) is obtained by reducing the particle size of beta-MnO2 nanoflowers with well-defined morphologies, which can be achieved by regulating the molar ratios of the reactants. In view of its easy preparation, the as-prepared beta-MnO2 is considered as a promising candidate for high-performance supercapacitor electrode material. (C) 2018 Elsevier Ltd. All rights reserved.