A comprehensive experimental study was conducted to identify the differences of the kidney section carbon fibers and circular section carbon fibers in the surface characteristics of fibers and mechanical properties of composites. It was revealed that the kidney fibers with larger specific surface area have a better adsorption characteristic and higher impregnating performance compared with the circular fibers. Mechanical tests under interfacial shear, interlaminar shear, flexural and compressive load have shown that the kidney fiber/epoxy composites outperform the circular fiber/epoxy composites by a significant improvement of 23.5, 12.7, 7.5 and 4%, respectively. The tensile strength is nearly the same for the two composite systems, but the failure of kidney fiber/epoxy tensile specimens undergoes a cumulative damage progression process and is different from that of circular fiber/epoxy tensile specimens. Also, the kidney fibers have a more non-uniform distribution in composites and larger contact areas with each other compared with the circular fibers, and the mechanical properties of kidney composites have a larger coefficient of variation than those of circular composites. (C) 2007 Elsevier B.V. All rights reserved.