The effects of alloying elements on the microstructure and high temperature mechanical properties of Mg-Al alloys were investigated in this study. In order to improve the high temperature mechanical properties, Sr or Mm. was added to the Mg-9Al alloy. The effect of Sri on the Mg-9Al alloy was also examined since Sri was expected to improve the high temperature mechanical properties by forming the thermally stable Mg(2)Sn phase. The specimens used in this study were produced on a 320 ton cold chamber high-pressure die casting machine. The microstructures of the specimens were examined by optical and scanning electron microscopy and tensile and creep tests were performed at elevated temperatures. Tensile tests were carried out at room temperature, 150 degrees C and 200 degrees C using an initial strain rate of 2x10(-4)/sec. In addition, tensile creep tests were conducted at the stress levels of 50 MPa and 70 MPa. From the microstructure analyses of the specimens after heat treatment at 400 degrees C for 12 hours, it was found that most of the Mg(17)Al(12) precipitate dissolved into the matrix, while the thermally stable phases continued to exist. The high temperature mechanical properties of the Mg-9Al alloys were found to improve significantly with the additions of Sr, Mm and Sn, due to the formation of the thermally stable precipitates.