Magnesium alloys with different Sm contents, with research focus on ZK30 and AZ31, were prepared and their microstructures and thermal conductivities were investigated. Results of microstructure analysis showed that Sm element in the as-cast ZK series of magnesium alloys was mainly distributed along the grain boundaries in the form of (Mg, Zn)3Sm rare-earth phase. However, Sm element in AZ series of magnesium alloys was distributed over the grains and at grain boundaries in form of Al2Sm phase. The Al2Sm phase at the core acted as a nucleus to refine the grain of AZ series of magnesium alloys. The grain size of the as-extruded magnesium alloys was obviously smaller than that of the as-cast magnesium alloys. Moreover, the crushed secondary phases were arranged along the direction of extrusion. Results showed that thermal conductivity of magnesium alloys could be improved by increasing Sm content. Thermal conductivities of ZK series of magnesium alloys were obviously higher than those of AZ series of magnesium alloys. This was mainly attributed to the significantly smaller lattice distortion of ZK series of magnesium alloys compared to that of AZ series of magnesium alloys.