Microstructures and mechanical properties of Mg-6Al-1.2Y-0.9Nd magnesium alloy with Sm addition were investigated through OM, SEM, EDS and XRD. The results show that Sm combines firstly with Al to form dispersed, high-melting-point particles which are mainly Al2Sm phase. When the mass fraction of Sm is high from 1.5% to 2.0%, the acicular Mg12Nd phase appears in the alloy. With the increase of Sm content within the range from 0 to 2.0%, the tensile strength of alloys at ambient and elevated temperature firstly increases slightly, and then decreases; while the elongation decreases continuously. The tensile fracture of the alloy is quasi-cleavage failure with plastic characteristic.