The effect of Al content on the microstructural characteristics, 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 permanent mould designed to prepare the specimens used in this study. Tensile and creep tests were carried out for each Mg-Al based alloys. Results show that creep resistance decreased while tensile strength improved with increasing Al contents. Microstructural study revealed that morphology and distribution of non equilibrium Mg17Al12 phase is main factor determining the tensile and creep properties of as cast Mg-xAl-Zn alloys. The final purpose of this study is to construct the database of already used Mg-Al based alloy and to develop the heat resistant magnesium alloys to apply transportation systems such as automobile.