Abstract: The electrochemical characterization, the chemical components and morphology of PAN-based carbon fibers treated by electrochemical oxidation in aqueous ammonium bicarbonate were analyzed using cyclic voltammetry, XPS and AFM. The results indicate that the electrolyzing reaction of water and the electrochemical oxide reaction of electro active materials are the main reactions taking place on the fiber surface in aqueous ammonium bicarbonate. The contents of every functional groups contained oxygen are changed during the treatment process. Amide functional groups are introduced on the fiber surface by the reactions of NH^＋₄ and the active functional groups. The morphology of the fiber surface is more and more rough with the treatment time extending under the coupling action of oxidizing erosion and gas etching.