The microstructures of the as-cast Mg-5Sn alloys with 0.5wt%, 1wt%, and 2wt% Cu addition were characterized by XRD and SEM. The results show that besides α-Mg and Mg2Sn phase, Mg2Cu appears in the Cu containing alloys. The addition of Cu results in obvious grain refinement. With Cu content increasing, the amounts of Mg2Sn and Mg2Cu increase, but the sizes of these two intermetallics coarsen. The tensile test at the ambient temperature shows that when Cu content is in the range of 0.5wt%~1.0wt%, the Cu addition helps to improve the strength of the alloys; however, excessive Cu addition leads to the deterioration of the ultimate strength. The optimum mechanical property is attained at Mg-5Sn-1.0Cu alloy, where the UTS and the elongation reach 180 MPa and 12%, respectively. The behavior of the Mg-Sn-Cu alloy in the compressive creep at 175 °C at applied load of 35~75 MPa was also investigated, which shows that the Cu addition helps to improve the compressive creep resistance of Mg-Sn alloy.