A microporous carbon powder derived from pulverizing a polyacrylonitrile-based activated carbon fiber showed an excellent performance serving as electrodes for symmetric electric double-layer capacitors using sulfuric acid as the electrolyte. In comparison with conventional activated carbon powders derived from a phenol-formaldehyde resin, this fiber-derived carbon showed a large ultimate capacitance value and could still retain a high capacitance at high current rates. Quantitative characterization on the pore structure of these carbons was conducted. The electrochemical impedance spectra of the capacitors were adequately fitted to an equivalent circuit containing a Warburg element. The fiber-derived carbon was found to have a greater effective diffusivity for electrolyte transport due to its smaller tortuosity factor, which had a value of 4-7 times smaller than those of the conventional carbon powders. The results of oxygen chemisorption on the carbons suggested that the pore walls of the fiber-derived carbon were more populated with graphite-like crystallite edges. This feature would lead to a stronger specific adsorption and thus a higher double layer capacitance.