Activated carbon fibers have been prepared from conventional pitch based carbon fibers by a catalytic activation method. The pore structures of the resulting activated carbon fibers before and after washing with dilute acid as well as of the activated carbon fibers with and without cobalt impregnation were analyzed, and the differences discussed. The electrochemical performance of the activated carbon fibers as electrodes in supercapacitors was characterized by galvanostatic measurements, cyclic voltammetry and AC impedance spectrum analysis. The results showed that the surface of the carbon fibers activated by cobalt impregnation had more mesopores and macropores than those without cobalt impregnation. A more porous structure was also obtained by washing the fibers with dilute acid. The specific capacitance of activated carbon fibers washed with dilute acid and impregnated with cobalt was 197F/g, some 50% higher than that of those without cobalt impregnation. At a current density of 1500mA/g, the specific capacitance reached 163F/g, and after 2000 DC charge/discharge cycles, the specific capacitance increased by 23%.