Abstract: Transition-Al₂O₃ nanopowder was first prepared by the precipitation-coated method combined with the freeze drying technique by using AlCl₃·6H₂O, ammonia, ammonium chloride (NH₄Cl) as raw materials, and then characterized with X-ray diffraction (XRD), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), BET surface area and particle size analyzer.Coating precipitation surface with NH₄Cl, adding surface activator, washing with ethanol and freeze drying were used to decrease particle agglomeration.γ-Al₂O₃ nanopowder with an average diameter of 6.73 nm and γ-δ-Al₂O₃ nanopowder with an average diameter of 11.92 nm were obtained by calcination of the precursors at 600℃ and 950℃, respectively.Phase transformation of Al₂O₃，the principle of precipitation surface coated by NH₄Cl and the mechanism of anti-agglomeration via surface coating were also investigated.