In this article, the as-cast microstructures and mechanical properties of the Mg-4Y-1.2Mn-0.9Sc and Mg-4Y-1.2Mn-1Zn (wt%) magnesium alloys are investigated and compared. The results indicate that the Sc-containing alloy is mainly composed of alpha-Mg and fine particle-like Mg24Y5, Mn12Y, and Mn2Sc phases, while the Zn-containing alloy mainly consists of alpha-Mg and coarse Mg12YZn phases with a continuous network. Furthermore, the grains of the Zn-containing alloy are relatively finer than those of the Sc-containing alloy. In addition, the Sc-containing alloy exhibits relatively higher tensile properties at room temperature and 300 A degrees C than the Zn-containing alloy. However, the creep properties at 300 A degrees C and 30 MPa for 100 h for the Sc-containing alloy are relatively lower than those for the Zn-containing alloy.