The effects of the Sm element on the microstructure of Mg-3Y-2Zn-0.6Zr (w/%) casting alloys were investigated, and the mechanical properties of the alloys were also tested at room temperature and different high temperatures of (423, 473, 523 and 573) K. Due to a heterogeneous nucleation, more refined grains were obtained with an increment of the Sm element. The precipitated phase at the grain boundaries changed from a discontinuous distribution to a continuous mesh. The main phase compositions were a-Mg, W phase-Mg3(Y, Sm)2Zn3 and X phase-Mg12(Y, Sm)Zn. The Y atoms were partly replaced by Sm in the secondary phase. The proportion of the substitutional solid solution increased and Sm-containing phases were not found in the alloy. With the increasing Sm amount, the tensile strength and yield strength increased slightly while the elongation decreased significantly at room temperature. However, the tensile strength at high temperatures decreased at first and then increased. As the temperature increased, the fracture of the MS alloys changed from the main intergranular fracture to a transgranular fracture.