The effect of Al content on the castability, tensile and creep properties of Mg-xAl-Zn alloy was investigated. The molten Mg-(3~11)Al-Zn alloys of 700°C were poured into the mould designed to estimate castability. Tensile and creep tests were performed for the alloys. The results show that the castability increased with higher Al content. Creep resistance decreased while tensile strength improved with increasing Al amount. Our microstructure study revealed that morphology of Mg17Al12 is the main factor determining the tensile and creep properties of as cast Mg-xAl-Zn alloys. The final purpose of this study is to develop the heat resistant Mg alloys to be applied in transportation systems such as automobile. Further research will be conductedon the effect of heat treatment and additional alloying elements on the microstructure and properties of Mg-xAl alloys.