On applying heat treatments to the amorphous alumina-silica fibre (55% Al2O3, 45% SiO2), the mullite crystal is separated out, and the crystallinity depends on the heat treatment parameters. The crystallized alumina-silica fibre shows higher hardness than amorphous alumina-silica fibre and a remarkable stability to chemical reaction with molten metal. The primary interfacial reaction zone product was found to be 2MgO.SiO2.