Platelike -Al2O3 powder was obtained at 850 degrees C by thermal decomposition of ammonium aluminum carbonate hydroxide precursor, which was treated by hydrofluoric acid. The results showed the decomposition of the as-prepared precursors experienced via three steps, adsorption of physical water, dehydration of crystalline water, and decomposition of the precursor, and the activation energies of the above three steps were calculated using the Coats-Redfern method. Meanwhile, the intermediate of (NH4)(3)AlF6 obtained played an important role in decreasing activation energy of the decomposition and phase transformation. Moreover, the thermal conductivity of the epoxy resin filled with platelike alumina was improved obviously.