Abstract: The surface of PAN-based carbon fibers was modified by elect rochemical oxidation. The surface state ofmodified carbon fibers was studied using SEM, AFM, XPS and XRD. The relationship s of surface state of carbonfibers and it s tensile st rength and mechanical performance of carbon fiber reinforced PAN (CFRP) were discussed.The result s show that surface roughness of modified carbon fibers was 1.1 times higher than that of the unt reatedone. Carbon content s decrease by 9.7 %, oxygen and nit rogen content s increase by 53.8 % and 7.5 times respectively, and the hydroxyl and carbonyl content s also increase. The surface orientation index reduces by 1.5 %, and themicrocrystalline dimension also decreases, and the active carbon atoms of carbon fiber surface increase by 78 %. Thetensile st rength of carbon fibers decreases by 8.1 % due to corrosion process of the elect rochemical oxidative method. The physical and chemical properties of carbon fiber, surface are modified during this process, the cohesivenessbetween the fibers and resin mat rix is improved and the ILSS of CFRP is increased by 26 %.