Facile synthesis of nano-crystalline alpha-Al2O3 at low temperature was systematically investigated. An easily accessible and reproducible sol-gel strategy utilizing aluminum isopropoxide, absolute ethanol and nitric acid (or hydrochloric acid) as raw materials was designed. The thermal decomposition of the precursor (gel) and subsequent formation of alpha-Al2O3 were characterized by X-ray diffraction (XRD), nitrogen adsorption and desorption measurements, thermogravimetric-differential scanning calorimetry (TG-DSC) and Fourier transform infrared spectra (FTIR). During the process of subsequent low temperature heat treatment of precursor gel, the formation of alpha-Al2O3 was firstly observed at temperature as low as 400 degrees C without the appearance of other transition phases. However, the additional gamma-Al2O3 diffraction peaks appeared at 800 degrees C unexpectedly and then gradually disappeared in the further calcination stage up to 1000 degrees C. These phenomena could not be illustrated by the traditional alumina phase transformation theories. Herein, a hypothetical scheme was put forward to explain these unique phenomena met with in our investigation. Besides, the critical role of the acid modifier in lowering the phase transformation temperature of alpha-Al2O3 was also carefully studied. The results indicated that the increasing molar ratio of acid to aluminum isopropoxide and using nitric acid as modifier were in favor of the phase transformation from gamma- to alpha-Al2O3. (C) 2012 Elsevier B.V. All rights reserved.