Abstract: As the surface modifier of carbon fiber, silica sol were prepared using silane coupling agent, tetraethoxysilane(TEOS) as the precursor and p-toluenesulfonic acid(PTSA) as the catalyst. The single fiber/epoxy composite were prepared using the treated carbon fiber as the reinforcement. The carbon fibers were characterized by examination of the morphology and mechanical properties using SEM, TEM and tensile testing. Interfacial shear strength (IFSS) of the composite was measured by single-fiber fragmentation testing. The results show that a layer of the membrane-grain structure on the surface of the carbon fiber is formed in situ after being treated by the silica sol. The tensile strength of a single fiber increases from 2.41 GPa to 3.00 GPa. The interfacial properties of the composite is greatly improved and the interfacial shear strength increases by 51.41%.