The transformation of well-crystallized macrogranular gibbsite to boehmite has been studied by hydrothermal method, at 165℃, 175℃, 185℃ and 195℃ for 12 h, respectively. Samples have been characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) before and after autoclaving. The transformation of gibbsite to boehmite is a dissolution-precipitation mechanism, rather than a solid-state process. The rate of transformation of gibbsite to boehmite in water vapour was faster than the rate in water. The size of obtained boehmite crystal (4~7 μm) reduced to thirtieth of the macrogranular gibbsite (120~200 μm), indicating that well-crystallized macrogranular gibbsite can be used to produce particulate boehmite. Because boehmite firstly crystallized on the outside surface of gibbsite particles during the period of gibbsite to boehmite, which keep the pseudomorphism of gibbsite particles with the double-layer structure, the hydroxide particles of aluminum were obtained by controlling the temperature and reaction time. Moreover, the hollow particles of boehmite aggregation were perhaps formed.