Manganese dioxide has been synthesized by a new simple self-reacting microemulsion method. The synthesized MnO2 has been found to be amorphous structure containing a moderate amount of water by X-ray diffraction, Fourier transform infrared spectroscopy and thermogravimetric analysis. Particles in a spherical shape with about 4 nm in diameter have been observed by transmission electron microscopy. Cyclic voltammetic tests have been performed between -0.5 and 0.5 V versus Hg/Hg2SO4 in 1 mol L-1 Na2SO4 solution at sweep rates up to 50 mV s(-1). A specific capacitance value as high as 246.2 F g(-1) was obtained, which was much higher than 146.5 Fg(-1) of MnO2 prepared by chemical co-precipitation. After 600 cycles, only 6% decrease of specific capacitance was measured which indicated that such a material possesses good cycling property. (C) 2008 Elsevier B.V. All rights reserved.