Alumina precursors were prepared by hydrothermal treatment of alkoxide-derived alcogels. The crystalline structure of precursor beohmites and their microstructural change during heat treatment were examined and the specific surface area of the alumina precursors after heating was measured. The alu mina prepared by hydrothermal treatment at 270 degrees C retained high specific surface areas at high temperatures; e.g. 35.0, 8.3 and 5.4 m(2) g(-1) at 1200, 1400 and 1500 degrees C, respectively. The thermal stability of the aluminas depended on the hydrothermal temperatures. For excellent thermal stability, the following factors are necessary : (1) grain growth of beohmite as an alumina precursor, and a grain size of more than 20 nm for the (1 20) plane; (2) a crystallite size for the (200) plane exceeding that for the (002) plane; (3) an isotropic growth of the beohmite crystal. In the transition alumina region (less than or equal to 1200 degrees C), the thermal stability of the alumina is caused by raising the a transformation temperature, resulting from decreasing the number of grain boundaries by beohmite growth. In the a-alumina region (>1200 degrees C), inhibiting the three-dimensional grain growth achieves thermal stability, resulting from preservation of the anisotropic structure introduced into the beohmite.