The microstructure and mechanical properties of the ZM81 alloys with varying Sn contents subjected to the extrusion and T6 treatments were investigated by OM, XRD, SEM, TEM, hardness test and uniaxial tensile test at room temperature, respectively. The results show that after the addition of Sn element, an Mg2Sn eutectic phase is formed, which can refine the as-cast microstructure. Sn element has an obvious effect on refining the microstructure of as-extruded alloys by restricting the occurrence of dynamic recrystallization and restraining the grain growth during extrusion, and improves the mechanical properties. T6 treatments, especially double aging, can significantly increase the strengths of the extruded alloys. Among them, the ZM81-4Sn alloy with double aging exhibits an ultimate tensile strength of 416MPa, a 0.2% yield strength of 393MPa and an elongation of 4.1%. The microstructure characterization suggests that the high strengths of the peak-aged alloys are attributed to the combined precipitation strengthening of the fine and dispersed MgZn2 and Mg2Sn precipitates, and the pre-cipitates after double peak aging are finer than those after single peak aging.