Ultra-high-temperature resistant Si-Al-C fibers derived from polyaluminocarbosilane were prepared by the procedures of melt-spinning, air curing and heated at 1800°C. It was found that oxygen content in the cured fibers has great influences on the chemical composition of the Si-Al-C fibers and its crystalline behavior during sintering. When oxygen content of cured fibers was too high, crystallite grains in the fiber would grow up. Oxygen evolvement in the fiber during the preparation was studied in detail by chemical analysis. It was found that oxygen is liberated mainly as CO gas from 1300°C to1600°C.