The effect of 0.5, 1.5 and 4.5 (wt.%) Sm addition on microstructure and mechanical properties of extruded Mg-6Al-1.0Ca-0.5Mn alloys was investigated. With increasing the content of Sm, the Al₂Sm phase precipitates in the matrix and the amount of the phase increases but that of Mg₁₇Al₁₂ phase decreases. The grains of alloys are refined due to the sufficient occurrence of dynamic recrystallization during extrusion process. The result of tensile tests shows that the alloy containing 1.5wt.% Sm exhibits the best mechanical properties. The improved mechanical properties of the alloy are mainly attributed to grain refinement, the precipitation strengthening of a large number of Al₂Sm particles and the decrease of Mg₁₇Al₁₂ phase in the matrix.