Alumina aerogels were prepared by an acid-base two step sol-gel synthetic route using aluminum tri-sec butoxide as precursor and nitric acid/ammonia as acid/base catalysts, followed by drying with supercritical ethanol.This method affords low-density (50-120kg/m3) and high-surface-area (400-500m2/g) alumina aerogel monoliths with a mesoporous distribution and high thermal stability.The structure and texture of the alumina aerogels before and after heat treatments were characterized using TEM, FTIR, XRD, BET, DSC/TGA techniques.It is found that the as prepared aerogels contained pseudoboehrnite microstructures and were converted into γ-A12O3 and δ-A12O3 and the corresponding specific surface area retains up to 381m2/g and 183m2/g after heat treatment at 800℃ and 1000℃ respectively.The aerogels prepared with small amount of acetylacetone had the similar phase and specific surface area evolutions after heat treatment, but showed higher thermal stability.The effect of the amount of distilled water, ammonia and chelating agent acetylacetone on the microstructures of aerogels and the changes of specific surface areas and pore size under different conditions were also discussed.