Chemical vapor curing (CVC) method for polycarbosilane (PCS) fiber with unsaturated hydrocarbon vapor, cyclohexene, was studied and compared with air oxidation curing method. The analysis of composition and thermogravimetry of PCS fiber in the process of CVC was also characterized and an elementary reaction mechanism was discussed. The results show that Si-H bond in the molecular structure of PCS reacted during the cyclohexene curing process and the reaction degree increased with the rise of the curing temperature. Simultaneously, gel content of PCS fiber rapidly increased till PCS fiber became infusible. Si-H bond and Si-CH₃ bond broke and formed free radicals through the agency of cyclohexene radical while being heated up, and then formed the structure of Si-CH₂-Si, resulting in cross-linking between PCS molecules.