Nanostructured hydrous ruthenium oxide (RuO2·xH2O) was directly deposited on tantalum (Ta) substrates by cyclic voltammetry (CV) technology in RuCl3·xH2O aqueous solutions. The growth mechanism, revealed by energy dispersive X-ray spectroscopy (EDS) and scanning electron microscopy (SEM) techniques, is that Ru3+ ions are reduced to nanostructured ruthenium (Ru) particles during negative potential scanning, then are oxidized to RuO2·xH2O granules (also in nanometers) during the following positive scanning. The unique nanometer structure of RuO2·xH2O, rather than the conventional “mud-cracked” structure, enable itself a high specific capacitance of 730 F·g-1 at a scanning rate of 50 mV·s-1 and a very fast charging/discharging ability. The speci?c capacitance is still up to 700 F·g-1 when the oxide loading is up to 1.2 mg·cm?2. In addition, the capacitance retains about 100% after 100,000 charging/discharging cycles. All of these indicate that the nanosized RuO2·xH2O is an excellent electrode material for energy storage and conversion devices, such as supercapacitors