Abstract: Polycarbosilane (PCS) fibers cured in 1-hexyne vapor have much less oxygen than those cured by air. The composition and structure of PCS fibers transform during 1-hexyne curing process. The results show that the reaction degree of Si—H bond and gel content of PCS fibers rapidly increase when the curing temperature increases. Si—H and Si—CH₃ bonds in PCS are induced by 1-hexyne to cleavage and form Si-central radicals. Fully developed crosslinking fibers come into being through the combination of these radicals. And a small amount of hexyl is introduced to PCS structure during the reaction. The tensile strength of SiC fibers prepared is 2.79GPa and the oxygen content is 5wt%~6wt%. The fibers show better high temperature resistance than Nicalon fibers. After being exposed in Ar to 1300℃, the SiC fibers maintain 80% of the initial strength; after being exposed in Ar to 1400℃, they maintain 60% of the initial strength. And during the heat treatment from 1300℃ to 1600℃, the change of β-SiCs grain size is only 2.18nm.