The surfaces of carbon fibers were treated by allylcyanide and xylene/air/argon plasma polymers. The surfaces of untreated and treated fibers were examined by scanning electron microscopy. Electron micrographs of the surface of carbon fibers showed that the surface striations and surface roughness were changed increasingly on the fiber surface after plasma treatment. Surface energy determinations of the fibers were carried out from advancing contact angle measurements, together with tensile testing of the single fibers. A qualitative agreement between an increase in the strength of the plasma-coated fibers and an increase in the surface energy of the fibers was observed.