Microstructure and mechanical properties of Mg-4Y-xSm-0.5Zr (x = 1, 4, 8) alloys during thermo-mechanical treatments were investigated in this study. Mg-4Y-4Sm-0.5Zr alloy exhibits higher tensile strength but lower elongation than Mg-4Y-1Sm-0.5Zr alloy during the thermo-mechanical treatments. Large amount of intermetallic phases still remained at grain boundaries in Mg-4Y-8Sm-0.5Zr alloy after solution. These undissolved phases can strengthen the grain boundaries at temperatures higher than 573 K. But the room temperature mechanical properties of Mg-4Y-8Sm-0.5Zr alloy during the thermo-mechanical treatments were greatly weakened for the brittleness of these undissolved intermetallic phases.