Gamma-ray radiation was used to surface treat PAN carbon fibers. The efficiency of gamma-ray radiation was compared with air oxidation in terms of variations in the surface structure of carbon fibers and the mechanical performance of their composites. It was observed that the composites reinforced with the gamma-radiated carbon fibers showed higher interfacial adhesion strength and thus better flexural and shear properties than the composites reinforced with air-treated fibers. The observed higher content of carboxyl group on the surface of the gamma-radiated carbon fibers is likely to be responsible for the stronger fiber-matrix bonding. It is concluded that gamma-ray radiation is an effective approach of tailoring surface properties of carbon fibers. (c) 2005 Springer Science + Business Media, Inc.