Effects of Ca alone and Ca/Sr composite additions on microstructures, die castability, mechanical properties and creep properties of the Mg-Al based alloys are investigated for the purpose of the development of new Mg-Al-Ca-Sr alloy for automotive powertrain applications. Ca addition of more than about 1% to AM50 alloy significantly improves creep resistance but also enhance casting crack tendency. By the addition of approximately 0.2%Sr, such casting cracks are significantly suppressed, and besides creep resistance and mechanical properties increase. The improvement of creep resistance by Sr addition seems to be attributed to the suppression of grain boundary sliding due to the creation of thermally stable Al-Sr compound along the grain boundary and the suppression of discontinuous precipitation of β-Mg₁₇Al₁₂ phase.