The present study discussed the effects of electrolytes and pore surface area and/or oxygen functional groups of activated carbon fiber (ACF) electrodes on their electric capacity. The electric capacity was affected by the electro-conductivity of electrolytes and increased with increase in oxygen functional groups. It depended more on oxygen functional groups than on BET surface area. However, ACFs which have higher acidity exhibited more remarkable decrease in capacity with current density, due to the increase of internal resistance. At the greatest 30% of increase in electric capacity was obtained by using ACFs with different amount of oxygen functional groups between the positive electrode and the negative one.