Mg-0.5%Zn-4%Al-0.5 similar to 2.0%Ca-0.5 similar to 2.0%La alloys are investigated in order to determine the optimum composition with superior casting properties and excellent creep resistance for automatic transmission case applications. The results show that regardless of the La content, the hot-tearing susceptibility of alloys containing 0.8%Ca or more is low. However, at 0.5%Ca content, hot-tearing occurs in alloys containing more than 1.2%La. The creep resistance of the alloys increases as the Ca content increases and Mg-0.5%Zn-4%Al-0.9%Ca-0.8%La alloy is found to be the optimum alloy exhibiting good casting properties and adequate heat, resistance for automatic transmission cases.