γ-Al₂O₃nanopowder whose specific surface area and average particle diameter is 189.6m²/g and about 9nm, respectively, is synthesized via sol-gel process using AlCl₃?6H₂O and (CH₂)₆N₄ as starting materials. Active Al₂O₃-SiO₂ composite powder is synthesized via diphasic sol-gel process using SiO₂and Al₂O₃ sol. After having been calcined at 1300℃ for an hour, Al₂O₃-SiO₂ composite powders transforms to mullite.