Electrochemical surface treatment of PAN-based carbon fibers in acidic electrolyte has been studied in increasing the surface acidic functional groups on fibers for the improvement of composite interfacial adhesion. With a viewpoint of acid-base interaction chemistry, low current densities are need for the changing of morphological and mechanical properties, This study reveals that anodic treatment under weakly oxidizing conditions in phosphoric acid electrolyte for 10 sec leads to surface modification of the fibers, preferentially prismatic planes. According to acid value, XPS and interfacial mechanical property measurements, it is revealed that the oxygen functional groups on fibers are largely dominated in composite mechanical behavior, whereas the nitrogen functional groups are not affected in the system studied. Also, a good correlation between surface acidic functionality and mechanical properties is established, and it is found that 0.4 A.m(-2) current density is the optimum conditions for this system.