A two-step surface treatment was applied on the surfaces of polyacrylonitrile(PAN)-based carbon fibers (CFs) including the electrochemical oxidation process followed by grafting of silane coupling agent KH550. The evolution of the physiochemical structure of the fiber surface during the surface treatment was investigated using Infrared spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and surface tension/dynamic contact angle measurements. The results showed that the CF surface exhibited a significant increase in the relative content of oxygen-containing functional groups after surface treatment while its crystal structure had little change after the grafting of KH550 agents. Effects of surface treatment on the mechanical properties of CFs and the interfacial properties, including the interfacial shear strength (IFSS) and interlaminar shear strength (ILSS) of carbon fiber reinforced epoxy (CF/EP) composites were evaluated in detail. Both the electrochemical oxidation and the introduction of KH550 agents onto CF surfaces improved the IFSS and ILSS values of CF/EP composites by comparison with untreated fibers. In particular, the maximum interfacial properties of CF/EP composites were obtained by the electrochemical oxidation together with the grafting of KH550 agents onto CF surfaces.