The effects of zirconium on the microstructure and mechanical properties of Mg-5Sn alloys were studied. It can be found that the grain of alloy is obviously refined with the addition of Zr. The mechanical properties experiments indicate that the tensile strength and especially the elongation of the alloys are improved gradually with rising of Zr content. The main reason is attributed to the grain refinement. The micro-hardness of the alloy changes little after the addition of Zr, which is due to no obvious appearance of the second hardening phases in the alloys.