The reaction process of polycarbosilane (PCS) fiber cured by cyclohexene vapor has been studied and compared with that of PCS fiber cured by air. The influence of curing temperature on Si-H bond reaction degree and gel content, the structure and composition of PCS were investigated by FTIR, EA, TGA, NMR, and GC-MS. The results showed that, Si-H bond in the molecular structure of PCS reacted during cyclohexene Curing process and the reaction degree increased when the curing temperature increases. Simultaneously, gel content of PCS fiber rapidly increased till PCS fiber became infusible. Si-H radical and Si-CH2 radicals formed Si-CH2-Si crosslinking of PCS molecules through the agency of cyclohexene. Some cyclohex ls linked to principal chain of PCS, which was proven c by C-13-CPMAS-NMR, and broke off, and cyclohexane and some monosilane are generated as byproducts when temperature increased. (c) 2007 Wiley Periodicals, Inc.