Abstract: Activated carbon was prepared from petroleum coke by chemical activation. Multi-walled carbon nanotubes (MWCNTs) were mixed with the activated carbon to form the electrode material of electric double layer capacitors. According to the relative values of impedance and capacitance in electrochemical impedance, the available capacitance and energy dissipation from resistance were evaluated. Results showed that the real part of the capacitance for this mixture was higher than for activated carbon alone when activated carbon was mixed with a mass fraction 3%-15% MWCNTs. With the increase of MWCNT content, the imaginary part of the capacitance and its percentage decreases while the real part of capacitance and its percentage increases. It was revealed that the efficiency of energy storage was improved, the relaxation time constant was decreased, the characteristic of capacitance response versus frequency was improved and the electrode resistance was decreased by the addition of MWCNTs to activated carbon.