In order to develop large-scale (100 mm in diameter) Mg-Zn-Y alloys with high strength beyond that of 4032-T6 alloy, we considered a novel casting process to produce large ingots with homogeneous microstructure, and investigated the mechanical properties of the extruded alloys. First, ingots (335 mm in diameter and 850 mm in height) were produced by a novel stir casting process in the ingot case. Then large-scale extruded alloys (100 mm in diameter) were prepared with an extrusion ratio of 10. The Mg-Zn-Y alloys exhibited higher yield and fatigue strengths than those of 4032-T6 aluminum alloy. The yield strengths of the aluminum alloy decreased drastically above 473 K, whereas those of the Mg-Zn-Y alloys did not. It is noteworthy that the yield strength (274 MPa) and fatigue strength (100 MPa) of the Mg-Zn-Y alloys at 473 K were about 1.3 and 1.2 times respectively as high as those of aluminum alloys. Moreover, creep strengths were equivalent to or higher than those of aluminum alloys. From the results above, we verified that even large-scale extruded Mg-Zn-Y alloys have higher strength than those of heat-resistant aluminum alloys. In terms of physical properties, the material is low-density, with a density approximately 70% that of an aluminum alloy. In addition, its thermal conductivity, an issue when a material is used in components, was higher than those of commercial magnesium alloys (AZ91D, WE54 et al.), and close to those of commercial aluminum alloys. We expect that Mg-Zn-Y alloys will be utilized in the heat-resistant automobile and motorcycle components category.