The effect of cation on the performance of activated carbon (AC)/graphite capacitor has been studied. Electrolytes using Li+ and four kinds of quaternary alkyl ammonium cations have been picked up for this investigation. The relationship between the ionic radii of cations and the specific capacitance of AC negative electrode in corresponding electrolytes has been correlated. The state of Li+ dissolved in PC (propylene carbonate) solvent has also been clarified.