Abstract: High strength PAN-based carbon fibers were continuously electrochemically oxidized by applying current to the fibers serving as an anode in aqueous NH4HCO3. Graphite crystallites on the surface of electrochemically oxidized carbon fibers were investigated by X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy and conductometric titration. It was found that after the electrochemical oxidation, the total amount of oxygen-containing groups on the fiber surface was increased by 8.54% and the amount of water-absorption on the surface increased by 5.34%. This suggests a surface property change from hydrophobic to hydrophilic, and as a result, the interlaminar shear strength of polymers reinforced with these fiber was significantly increased. A linear relationship between crystallite size (Lc, La) and current density was also found.