Polyacrylonitrile (PAN)-based, microporous activated carbon fibers (ACFs) were investigated as electrode materials for electrochemical capacitors in KOH electrolyte solutions. PAN–ACFs were analyzed by using nitrogen adsorption data and the Dubinin–Raduskevich equation. The pore size distributions were narrow. As the micropore volume increased, the average micropore diameter decreased. Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and charge–discharge profile analysis were used to investigate the penetrability of electrolyte ions into the carbon electrodes. The electrolyte ions in the carbon electrode with a large micropore diameter could be accessible easily even if the pore size distribution is narrow. However, the specific capacitance decreased in spite of the increase of the micropore volume.