By using melamine as a nitridizing reagent, aluminum nitride (AlN) powders were prepared from δ- and α-alumina (Al2O3) powders which had not been mixed with carbon. The hydrogen cyanide (HCN) formed by thermal decomposition of melamine was involved in the nitridation of Al2O3 to AlN. The nitridation process of alumina to AlN was monitored by powder X-ray diffraction patterns and 27Al magic-angle spinning nuclear magnetic resonance spectroscopy. δ-Al2O3 began to be converted to AlN at 1050°C without δ-α alumina transformation. The nitridation rate of δ-Al2O3 was much higher than that of α-Al2O3.