The surface of carbon fibers were modified by plasma polymerization of different monomers such as dioxane and xylene in a radio frequency sputtering device. Argon, as carrier gas, and air, as a reactive component, were used under different plasma conditions. The operating conditions for plasma treatment were first optimized to yield the highest deposition rates with minimum power input. The topography of carbon fibers as well as of fracture surfaces of carbon fiber/epoxy composites surfaces, before and after plasma treatments, were investigated by scanning electron microscopy (SEM). The surface of untreated and treated carbon fibers was examined by using ESCA. The plasma polymers obtained were characterized by FTIR spectroscopy. The effects of plasma treatment on the surface of carbon fibers and on the mechanical properties of carbon fiber/epoxy composites were investigated. Dioxane and xylene plasma coating were found to be effective in enhancing the interfacial adhesion between carbon fiber and epoxy matrix as well as increasing the tensile strength of carbon fibers.