In this study, corncob-based activated carbons (ACs) were prepared by carbon dioxide (CO2) activation and their use as electrodes in the electrochemical double layer capacitor (EDLC) performed successfully. The effect of activation time (1-7 h) on the surface area and pore structure was detected by nitrogen adsorption-desorption isotherm at 77 K. The electrochemical properties of these corncob-based ACs were evaluated by using cyclic voltammetry (CV), galvanostatic charge-discharge and electrochemical impedance spectroscopy (EIS) studies. The results showed that the surface area increased to the maximum of 757 m(2) g(-1) with the prolonged activation time. However, when the current density was 50 mA g(-1), the specific capacitance reached to the maximum of 178 F g(-1) in 6 mol L-1 KOH solution when the activation time was 5 h. The capacitance was still up to 157 F g(-1) when increasing the current density to 1000 mA g(-1), indicating a high-rate charge-discharge performance. Furthermore, compared with other reported biomass based ACs, corncob-base ACs showed significantly higher surface specific capacitance (SSC) which means the accessibility and efficient utilization of surface area of these ACs.