The thermal stability of carbon fibers coated with different metals was investigated. The fracture strength of metal-coated carbon fibers was measured at room temperature as a function of heat-treatment temperature. It was demonstrated that the fracture strength of the copper-coated carbon fibers (C/Cu) was not affected by heat-treatment at temperatures up to 900 degreesC because of the inert of copper. However, the fracture strength of the carbon fibers coated with an active metal (nickel, iron and chromium) were found to decrease significantly after heat-treatment at temperatures > 700 degreesC, owing to graphitization (induced by nickel) of the carbon fibers or a chemical reaction (between fibers and iron or chromium) at the interface. A diffusion barrier layer between the active metal coating and the fibers can reduce the strength loss of the carbon fibers, but the effectiveness of a metal diffusion barrier differed depending significantly on the nature of the introduced metals. It was found that if the diffusing active metal had an adequate solubility in a metal, the latter can effectively hinder the diffusion of the former; if not, the latter can not effectively hinder the diffusion of the former.