Phase transformation of Mg vapor was simulated according to thermal dynamics during the fabrication of AlN composite in directed metal nitridation process, which further proves the presence of Mg vapor layer over the melt. Therefore, the thickness of Mg vapor layer was investigated using the principle of gas flow diffusion dynamics. The result shows that the thickness of Mg vapor layer is related to the distance from molten surface to upper fringe of crucible, oxygen partial pressure and original Mg concentration. With the decrease of oxygen partial pressure or increase of original Mg concentration, the thickness of Mg vapor layer increases, which benefits the penetration and nitridation of Al melt.