Mg-8%Al-1%Ca was selected as a base alloy composition and small amounts of minor alloying elements, RE or Sr, were added. Microstructure of as-cast Mg-8%Al-1%Ca base alloys consists of dendritic primary Mg, Mg-Al-Ca and Mg17Al12 phases. Mg-Al-RE-(Ca) phase in the RE-added alloys and Mg-Al-Sr-(Ca) phase in the Sr-added alloys were additionally found. The creep resistance of Mg-8%Al based alloy was significantly improved by the minor alloying elements additions with 1%Ca. Comparatively globular primary phase could be obtained after just 30min. through an isothermal heating of as-cast Mg-Al-Ca alloys.