The electrochemical properties of isotropic and anisotropic carbon fibers were studied using the two-electrode method in a 1 M LiPF6 electrolyte solution dissolved in a 1:1 volume mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). Isotropic carbon fiber had a larger irreversible capacity than that of the other carbon fibers and might have many sites for the electrolyte decompositon and residual lithium ions. Active carbon fibers prepared from isotropic carbon fibers by steam gasiformation at 850 degrees C showed large specific surface areas. After steam gasiformation, the active carbon fibers changed into a more crystalline state and had electrochemical characteritics similar to the anisotropic carbon fibers. Impedance profiles of the carbon fibers were fitted by equivalent analogs of a three-resistance-capacitance (RC) circuit in series and the thickness of passivation layers in the carbon fibers was calculated.