The effect of the improvement of commercial activated carbon(AC) on its specific capacitance and high rate capability of double layer(dl) charging/discharging process has been studied. The improvement of AC was carried out %via% a secondary activation under steam in the presence of catalyst NiCl-2, and the suitable condition was found to be a heat treatment at about 875 ℃ for 1 h. Under those conditions, the discharge specific capacitance of the improved AC increases up to 53.67 F/g, showing an increase of about 25% as compared with that of as-received AC. The good rectangular-shaped voltammograms and A.C. impedance spectra prove that the high rate capability of the capacitor made of the improved AC is enhanced significantly. The capacitance resistance(RC) time constant of the capacitor containing the improved AC is 1^74 s, which is much lower than that of the one containing as-received AC(an RC value of 4.73 s). It is noted that both kinds of AC samples show a similar specific surface area and pore size distribution, but some changes have taken place in the carbon surface groups, especially a decrease in the concentration of surface carbonyl groups after the improvement, which have been verified by means of X-photoelectron spectroscopy. Accordingly, it is suggested that the decrease in the concentration of surface carbonyl groups for the improved AC is beneficial to the organic electrolyte ion penetrating into the pores, thus leading to the increase in both the specific capacitance and high rate capability of the supercapacitor. The effect of the improvement of commercial activated carbon (AC) on its specific capacitance and high rate capability of double layer (dl) charging / discharging process has been studied. The improvement of AC was carried out % via % a secondary activation under steam in the presence of catalyst NiCl - 2, and the suitable condition was found to be a heat treatment at about 875 ° C for 1 h. Among these conditions, the discharge specific capacitance of the improved AC increases up to 53.67 F / g, showing an increase of about 25% as compared with that of as-received AC. The good rectangular-shaped voltammograms and AC impedance spectra prove that the high rate capability of the capacitor made of the improved AC is significantly significantly. The capacitance resistance (RC) time constant of the capacitor containing the improved AC is 1 ^ 74 s, which is much lower than that of the one containing as-received AC (an RC value of 4.73 s). It is noted that both kinds of AC samples show a similar specif ic surface area and pore size distribution, but some changes have taken place in the carbon surface groups, especially a decrease in the concentration of surface carbonyl groups after the improvement, which have been verified by means of X-photoelectron spectroscopy. suggested that the decrease in the concentration of surface carbonyl groups for the improved AC is beneficial to the organic electrolyte ion penetrating into the pores, thus leading to the increase in both the specific capacitance and high rate capability of the supercapacitor.