High surface area activated carbon (AC) obtained by KOH activation of petroleum coke was leached by HCl aqueous solution and the effects of the ash contents of AC on performance of electric double layer capacitors (ECs) in 3M KOH aqueous electrolyte and 1 M (C2H5)4NBF4/propylene carbonate electrolyte were investigated by cyclic chronopotentiograms and electrochemical impedance spectrometry (EIS). An equivalent circuit model based on a mixed kinetic and charge transfer control was proposed to fit EIS spectra. The model parameters were extracted by Marquardt best fitting. The series solution resistances, polarization resistance and diffusion coefficient related to Warburg impedances were obtained. It is found that the series solution resistances remained constant, polarization resistances decreased and the diffusion coefficients increased after acid leaching for each electrolyte, indicating that acid leaching can remove the electronic inert ash, which decreases contact resistance among particles of activated carbon and opens or widens the blocked pores by ash components. Moreover, the specific capacitances of AC were increased by the acid leaching.