Aluminum nitride (AlN) powder was easily synthesized by the direct nitridation of Al melt containing ~20 wt.% Mg catalyst and the nitriding behavior was investigated by thermodynamic calculation and through observations of electron microscopy and X-ray diffraction. The addition of Mg catalyst decreased the nitriding temperature below <TEX>$1,000^{\circ}C$</TEX>, which is comparable to the high nitriding temperature of <TEX>$1,400^{\circ}C$</TEX> required in carbothermal method. It was caused by a significant increase of the solubility of nitrogen gas due to the increase of Mg catalyst in Al melt. The dissolved nitrogen gas met Mg catalyst and was transformed into metastable <TEX>$Mg_3N_2$</TEX>. Finally the metastable phase reacted with Al to AlN.