Abstract: Rayon fibers were impregnated by an organosilicone additive followed by heat treatment and the structure and properties were investigated. Results show that strength of the heattreated rayon fibers impregnated by the organosilicone was improved The number of pores in the resulting carbon fibers was decreased and the separability of filaments was improved because the organosilicone may protect the surface of the fibers during heat-treatment by virtue of its heat-resistant, non-conglutination, hydrophobic nature and chemical inertness. When the loading of the organosilicone additive on the rayon fibers was 4.79 w/%, the tensile strength of the resulting carbon fibers was increased by 126.3%. The organosilicone additive was decomposed to form SiO2 after heat treatment.