Abstract: It is obtanred that the properties (σ_f,Cox/Cgr,surface defect) of two kinds of fiber, PAN-based and pitch-based carbon fiber, change with the variation of anod surface conditions in this work.The chemical activity and physical activity of carbon fiber surface can be greatly increased by anod surface treatment.The experiment results show that four kiads of carbon-oxide a.nd the tiny surface defect can be formed on the surface of carbon fibers by and surface treatment.The ratio C_ox/C_gr,the relative total content of carbon-oxide,and the size and amount of tiny surface defect increased with the increase of anodic potential.The shape of surface defect of PAN-based carbon fiber is different from that of pitchbased carbon fiber.It is observed that the anodic potential is the most important parameter for anod surface treatment of carbon fibers.