Abstract: The study of the mullite formation of two types amorphous alumina-silica fibers was performed with the help of heat treatment.The change in the distribution of Al⁺³,Si⁺⁴ and O^-2 on the cross section of fibers was studied. The effcet of the mu111tephase on the vacuum-filtration formability of fiber preforms and its mechanism were investigated. The experimental results reveal that the mullitephase of aluminasilica fibers reaches approximately 50 mass-% with the help of heat treatment;rest, amorphous.In the process of crystallization,the change in the distribution of Al⁺³,Si⁺⁴ and O^-2,on the measured layer of approx.400 Å from the surface of fibers was cornpanied. In the thin layer Si⁺⁴,namely SiO₂,was concentrated. It resulted in that the tensile strengtlis of the single fibers decrease about 2/3 of that of original fibers,After the heat treatment and mechanical chopping and wet process, the average fiber lengths decrease greately and the roughness R.of the free forming surface of ring preforms klowers distinctly,in other words,the inside surface of ring preforms gets smoother and circular. The vscuum-filtration formability of the heat treated alumina-silica fibers is much better than that of original fibers and it results from the crystallization of the fibers.