The effects of precipitates and alloying element on the high temperature tensile properties of Mg-Al alloys were investigated in this study. In order to modify the precipitation behavior and microstructure of Mg-Al alloys, Sr and/or Mm (Misch metal) were added to the Mg-9Al and AZ91 alloys. All test specimens used in this study were die-cast on a 320 ton cold chamber high-pressure die-casting machine and the microstructures of the specimens were examined by optical and scanning electron microscopy. Tensile tests were carried out at room temperature, 150oC and 200oC at a strain rate of 2×10-4/sec. The microstructure analyses revealed that thermally stable MgAlSr and AlMm compounds were precipitated in the Sr and/or Mm added alloys and these compounds suppressed the precipitation of the discontinuous Mg17Al12 phase at grain boundaries. The high temperature mechanical properties of the Mg-Al alloys were found to increase with the addition of Sr and/or Mm. It was concluded that the addition of Sr and/or Mm improved high temperature tensile properties of the Mg-Al alloys by the formation of thermally stable precipitates.