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A thorough understanding in thermal evolution of alumina hydroxide precursor is fundamental to prepare high-purity alumina powders though a sol-gel process. This work contributes this by investigating the structural evolution of boehmite precursor after annealing at a wide temperature range, 300~1400 °C in air for 1 h. TG-DSC analysis revealed the transformation process of dry boehmite gel. With heat-treatment up to 1400 °C, structural evolution of dry boehmite gel was carried out by combination of XRD, FT-IR, NMR and TEM analysis. The results show that the γ-Al₂O₃ is formed from boehmite at 500 °C, which transform into δ-Al₂O₃ up to 1000 °C. After sintering at 1100 °C, the appearance of a three-phase region including δ-, θ- and initial α-Al₂O₃ is observed. The complete transformation into α-Al₂O₃ lasts until at 1200 °C. The α-Al₂O₃ powders prepared with so-gel method at 1200 °C for 1h had an average particle size of ≈50 nm with good dispersivity and narrow particle size distribution.