The effect of Ga on the microstructure and mechanical properties of Mg-8wt.%Al alloy at room temperature were investigated by SEM and X-ray analysis. The microstructure of as-cast Mg-8wt.%Al alloy was consist of α-Mg matrix, β- Mg17Al12 and α+β eutectic phases. The primary β- Mg17Al12 phase precipitate along the α-Mg matrix grain boundary and in the discontinuous reticular shape. Ga addition in the as-cast Mg-8wt.%Al alloy change the microstructure significantly. When the addition is less then 2% Gd could be detected and distributed mainly within β- Mg17Al12 phase and in the interface of α-Mg matrix and β- Mg17Al12. With increasing Ga content α-Mg matrix grains and β- Mg17Al12 phase are refined. The amount of α+β eutectic structure increase as well. The holes within β phase are diminished. When Ga content was more than 2% with increasing of Ga content β phase was broken along the concentration of Ga. With the increase of Ga content to 4%, a new phase of Ga2Mg form in the microstructure and β phase precipitate in the shape of smooth baboon along the grain boundary. The mechanical properties test showed that as-cast Mg-8wt.%Al-2wt.%Ga alloy has the optimum combination of ultimate tensile strength of 248 MPa and elongation of 10%.