An anodic surface treatment of PAN-based carbon fibers has been studied in increasing the surface functional groups on fibers, resulting in increasing composite mechanical behavior. With a viewpoint of acid-base interaction chemistry, the 10 wt% phosphoric acid solution is selected for the electrolyte. As an experimental result, very low electric current densities, such as 30-300 mu A.m(-2), are need for the changing of morphological and mechanical properties. According to FT-IR and XPS measurements, it reveals that the oxygen functional groups on fibers are largely dominated in ILSS of the composites, whereas the nitrogen functional groups are not affected in this system. Also, it is found that a moderate 70 mu A.m(-2) treatment on fibers seems promising to assess the improving of the physical and mechanical properties.