The variation of solid–liquid interfacial energy (σ) for Mg–Al binary alloys was investigated as a function of Al content (3, 6, and 9 wt pct) based on the microstructure analysis of directional solidified Mg alloys. Primary dendrite arm spacing was measured from the directionally solidified alloys and used in Kurz and Fisher’s and Trivedi’s relations to calculate the value of σ. The calculated results reveal that the increasing Al content in Mg can significantly decrease the interfacial energy, which indicates a possible high adsorption tendency of Al at the solid/liquid interface.