Synthesis and thermal decomposition of ammonium aluminum carbonate hydroxide (AACH) have been studied. IR, TG, DTA and XRD results showed that, although AACH aged under different pH might have difference in their composition, the crystal structures are the same. The morphology of AACH and the alumina heated to different temperature were observed in an environment TEM, and no dramatic change has been found during the course. BET results showed the specific surface area of alumina decreased with the increasing heating time and temperature. Size distributions of crystal were measured by using a synchrotron radiation small-angle X-ray scattering (SAXS), the distribution peak shifted to smaller diameter as AACH decomposed to gamma -alumina. then theta -alumina.