Abstract: Activated carbons were prepared from corncobs by an initial hydrothermal treatment using ammonium chloride as a pore forming additive and then by KOH activation. Samples were characterized by SEM, XRD, TG analysis and nitrogen adsorption. The effects of preparation conditions on the iodine number, yield and their electrochemical performance as supercapacitor electrodes were investigated. Results indicate that NH₄Cl is helpful for the formation of activated carbons with a hierarchical pore structure. The best activated carbon prepared has a good rate performance with a capacitance of 175 F·g^-1 at 0.5 A·g^-1 and a capacity retention ratio of 73.0% at 20 A·g^-1, and a good cycling stability with a specific capacity change from 175 to 173 F·g^-1 after a 10 000 charge/discharge test.