The thermal decomposition and the phase transformations of ammonium aluminum carbonate hydroxide (AACH) to α-alumina have been studied. Amorphous-, γ- and θ-alumina were identified as intermediate products. A TTT diagram for the process of the thermal decomposition and the phase transformations was determined. Kinetic analysis of the θ- to α-aluminas transformation by the Avrami-Erofeev equation showed that the transformation occurred by the nucleation and growth mechanism. Results of transmission electron microscopy (TEM), however, suggested that the transformation occurred by the synchro-shear mechanism. The maximum particle size of θ-alumina and the minimum particle size of α-alumina were approximately 30 and 40nm, respectively. A model of the transformation was presented from the viewpoint of crystallography.