The medium-range structures of Si-Al-C-O, Si-Zr-C-O and Si-Al-C Tyranno fibers, which were prepared by pyrolysing polymetalocarbosilane organic fibers, were observed by means of small angle X-ray scattering using a point-collimated Cu Kalpha incident beam and a two-dimensional imaging plate detector. Single Si-Al-C-O and Si-Zr-C-O Tyranno fibers of a few mum in diameter have an anisotropic structure comprising thousands of fine filaments of about 10 nm in diameter bundled together along with the long axis of the fibers. The anisotropic structure is not sensitive to the pyrolysing temperature during the organic-to-inorganic conversion process. However, the anisotropic structure of the Si-Al-C-O fibers is totally modified to an isotropic one in Si-Al-C fibers prepared by heating the Si-Al-C-O fibers above 1700degreesC in an argon gas stream, because of the formation of an aluminum oxide-rich phase at the grain boundaries of beta-SiC nanoclusters.