Ozone method was used to modify surface activity of carbon fiber to improve the compressive strength and flexural strength of carbon/carbon composites. The SEM photos of composites fracture shows that ozone method treatment of carbon fiber improved the interfacial adhesion between fibers and matrix, which was the reason of increasing the compressive strength and flexural strength of the carbon/carbon composites. The untreated and treated carbon fibers were investigated by AFM, XPS and the micro-laser Raman spectroscopy, and the contact angles were used to analyze the surface properties of carbon fibers. The results indicated that oxidation of ozone method increased the carbonyl functional group and the surface roughness, changed the graphitization degree of carbon fiber surface and improved the wettability of carbon fibers, so that the compressive strength and flexural strength of the carbon/carbon composites were enhanced. The stronger wettability decreased the formation of traps in the composites, which was one of the major reasons that improved the compressive strength and flexural strength of carbon/carbon composites. (c) 2005 Elsevier B.V. All rights reserved.