Abstract: Surface treatment of isotropic pitch-based carbon fibers by electrochemical oxidation was carried out, and the processes of electrochemical oxidation were investigated through Atomic Force Microscopy (AFM) and X-ray Photoelectron Spectroscopy (XPS). The XPS studies indicated that the surface carbon and functional groups were progressively oxidized into carboxyl and carbon dioxide. The analyses of AFM images and surface roughness showed that the surface treatment first made carbon fiber smoother, and then caused striated surface after continual treatment. AFM illustrated the surface form on a microscopic scale effectively compared to SEM. Combining XPS with AFM can characterize the process of electrochemically oxidized pitch-based carbon fiber.