A novel solid state route has been successfully developed for the synthesis of nano-scale hydrous ruthenium oxide (denoted as RuO2·xH2O). The procedure involves directly mixing RuCl2·xH2O with alkali to form RuO2·xH2O in a mortar at room temperature. Transmission electron microscopy (TEM) and N2 adsorption–desorption measurement indicate that the RuO2·xH2O particle is approximately 30–40nm with mesoporous structure. The crystalline structure and the electrochemical properties of RuO2·xH2O have been systematically explored as a function of annealing temperature. At lower temperatures, the RuO2·xH2O powder was found in an amorphous phase and the maximum capacitance of 655Fg−1 was obtained by annealing at 150°C. Higher temperatures (exceeding 175°C) presumably converted amorphous phase into crystalline one and the corresponding specific capacitance dropped rapidly from 547Fg−1 at 175°C to 87Fg−1 at 400°C. Also, the dependence of electrochemical performance on annealing conditions of RuO2·xH2O was investigated by electrical impedance spectroscopy (EIS) study.