Polycarbosilane (PCS) was modified by addition of 1 wt% polyborazine and subsequent heat treatment at 70-400 ℃. The modified PCS exhibited highly improved ceramic yield from original 63% to 78% by heating at 70 ℃ for 10 h. The molecular weight of the PCS modified at higher temperatures markedly increased, then was rendered infusible but soluble PCS in organic solvent on heating at 300 ℃. (1)H-NMR indicated that original PCS was enhanced by dehydrocoupling reaction between Si-H groups. Solution of the modified PCS was hand-drawn to fibers, followed by direct pyrolysis up to 1,400 ℃ without air-curing step to convert into oxygen-free SiC fibers. The morphology and crystalline phase were determined by SEM and XRD analysis, respectively.