Abstract: α-Al₂O₃ nanocrystalline powders were synthesized by calcining the dry Al（OH）3 gels prepared from aluminum nitrate and ammonia solution using precipitation method,and we studied TG/DTA and the transformation of metastables Al₂O₃ in the process of calcining different dry Al（OH）₃ gels.Because of NH₄NO₃ and α-Al₂O₃ seed crystal,the nucleation barriers of θ→α reduced,consequently,the phase transformation temperature can be reduced for 40℃ by the effect of NH₄NO₃;and about 220℃ lower than the normal transformation temperature because of the dual role of the NH₄NO₃ and 2wt%α-Al₂O₃ seed crystal.The obtained α-Al₂O₃ powder has no obvious agglomeration,narrow particle size distribution,hexagonal spherical shape,and a mean particle size of 70nm.