Electron-beam-cured polycarbosilane fibers were heat-treated at. 673-1773 K in a tube evacuated to 1.3 x 10(-1) Pa and then exposed at 1873 K in argon, The effect of vacuum heat treatment on improving the high-temperature stability of low-oxygen SiC fibers was investigated by examining gas evolution, grain growth, surface composition, tensile strength, and morphology, The fibers heat-treated at <1173 K lost strength, because of the vigorous generation of residual hydrogen. A minute amount of oxygen in the atmosphere caused the active oxidation of SIC during heat treatment at >1673 K, resulting In severe strength degradation for the as-heat-treated fibers. Vacuum heat treatment at 1573 K provided the best characteristics in low-oxygen SiC fibers.