In order to discuss quantitatively the effect of functional groups of activated carbon fibers (ACFs) on the electric double-layer characteristics, ACFs with various amounts of functional groups were prepared from the same ACFs by redox methods. The amounts of functional groups of ACF were determined and compared using two different methods of back titration and X-ray photoelectron spectroscopy (XPS). In the case of the aqueous electrolyte, the electric capacity depended more on oxygen functional groups than on BET surface area. At the greatest 30% increase in electric capacity was obtained by using ACFs with different amounts of oxygen functional groups between the positive and negative electrodes. It was considered that a larger number of functional groups promote not only the wettability of electrodes but also the negative charge of electrodes leading to an increase in capacity. On the other hand, in the case of the organic electrolyte, the pore structure seemed to be a more dominant factor than functional groups. Also, the characteristics of the electrodes after 30,000-cycle charge-discharge were determined to confirm the reliability in an extended cycle operation.