The effects of Ce, Y and Gd additions on the as-cast microstructure and mechanical properties of Mg-3Sn-lMn alloy were investigated and compared by using optical and electron microscopies, X-ray diffraction analysis, and tensile and creep tests. The results indicated that the microstructure of the Mg-3Sn-1Mn alloy with the addition of 0.87 wt.%Ce was refined while the microstructures of the alloys with the additions of 0.79 wt.%Y or 0.84 wt.%Gd were coarsened. Furthermore, adding 0.87 wt.% Ce to the Mg-3Sn-lMn alloy could improve the tensile properties while adding 0.79 wt.%Y or 0.84 wt.%Gd gave a detrimental effects on the tensile properties. In addition, adding 0.87wt.%Ce, 0.79 wt.%Y and 0.84 wt.%Gd to the Mg-3Sn-lMn alloy could improve the creep properties. Among the three additions, the addition of 0.87 wt.% Ce was more suitable to further improve the strength and creep performance of the Mg-3Sn- 1 Mn alloy.