EB-cured PCS fibers were heat-treated at 1273-1673 K under a reduced pressure of 1.33 Pa, and subsequently they were exposed to 1773 K in air. The thermal stability and oxidation resistance of the fibers were investigated through TG, XRD analysis, specific resistivity measurement, SEM observation and tensile tests. The oxidation rates at initial stage are thought to be strongly dependent upon the microstructure of the fibers in the as-heat treated state and the presence of water vapor. Incomplete ceramization and the occurrence of active-oxidation during heat-treatment yielded poor oxidation resistance to the fibers. The oxidation caused the grain growth of SiC, drop of resistivity and degradation of strength. The oxidation of the fibers was retarded at later stage. The fibers heat-treated at 1573 K had high strength and high oxidation resistance. (C) 2003 Kluwer Academic Publishers.