Abstract: The effects of electrochemical treatment of carbon fibers in different aqueous phosphate were investigated. The Latin square experiment was used to optimize the treatment parameters. The chemical components, morphology and specific area of fibers surface were characterized by using XPS, AFM and BET technique.The results indicate that the specific current density and the electrolyte concentration are the significant factors in fiber surface treatment in (NH₄)₂HPO₄ and NH₄H₂PO₄ solutions, respectively. The effect of modification is very outstanding in (NH₄)₂HPO₄ solution because the oxidation and corrosion are moderate, and the ILSS of its composite is increased to 79.8 MPa. Whereas the effect of modification is difficult to control in NH₄H₂PO₄ solution due to the drastic oxidation and corrosion, the tensile strength of the bulk fiber is damaged greatly and the ILSS of its composites is increased only to 70.8 MPa.