In order to discuss quantitatively the effect of functional groups of activated carbon fibers (ACFs) on electric double-layer characteristics, ACFs with various amounts of functional groups were prepared from the same ACF by redox methods. The amount of functional groups of ACFs was compared between the method by back titration and by 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% 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. It was considered that a larger number of functional group promotes 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, pore structure seemed to be a more dominant factor than functional group. Also, the characteristics of the electrodes after 30,000-cycle charge–discharge were determined to confirm the reliability in an extended cycle operation. A significant increase in phenolic hydroxyl group was confirmed on the positive electrode.