The effect of Gd/Al ratio on the properties of as-cast Mg-Gd-Al-Zn alloys was investigated by changing the chemical composition from that of AZ61 to GZ61. At the ratio of 1, the Al_2 Gd phase becomes predominant and Mg_(17)Al_(12) is hardly seen in the microstructure. As a potent inoculant, the Al_2Gd phase resulted in intense grain refinement and enhancement of strength, ductility and toughness. For instance, the tensile strength and elongation to failure of Mg-3Gd-3Al-1Zn alloy were enhanced by ~4% and 180% compared with those of AZ61 alloy, respectively. However, at high Gd/Al ratios, the Al_2Gd phase was replaced by(Mg,Al)_3Gd and Mg5 Gd phases and very large grain sizes were achieved, which led to poor tensile properties and the appearance of cleavage facets on the fracture surfaces. Therefore, it can be deduced that the presence of Gd and Al, in appropriate amounts to reach Gd/Al ratio of ~ 1, is required for the achievement of grain refinement, good ductility, high strength, and the appearance of ductile fracture surfaces in the Mg-Gd-Al-Zn system.Conclusively, the Mg-Gd-Al-Zn alloys can be considered as a new class of structural magnesium alloy and it is superior to both AZ(Mg-Al-Zn) and GZ(Mg-Gd-Zn) series of alloys. The effect of Gd / Al ratio on the properties of as-cast Mg-Gd-Al-Zn alloys was investigated by changing the chemical composition from that of AZ61 to GZ61. At the ratio of 1, the Al 2 Gd phase becomes predominant and Mg_ As a potent inoculant, the Al 2 Gd phase resulted in intense grain refinement and enhancement of strength, ductility and toughness. For instance, the tensile strength and elongation to failure of Mg-3Gd However, at high Gd / Al ratios, the Al 2 Gd phase was replaced by (Mg, Al) _3Gd and Mg 5 Gd phases and very Large grain sizes were achieved, which led to poor tensile properties and the appearance of cleavage facets on the fracture surfaces. Thus, it can be deduced that the presence of Gd and Al, in an amount corresponding to reach Gd / Al ratio of ~ 1, is required for the achievement of grain refinement, good ductility, high strength, and the appearance of ductile fracture surfaces in the Mg-Gd-Al-Zn system. Confclusively, the Mg-Gd-Al-Zn alloys can be considered as a new class of structural magnesium alloy and it is superior to both AZ -Zn) and GZ (Mg-Gd-Zn) series of alloys.