To further increase the mechanical properties, 0.5wt.% Sm was introduced to a Mg-10Y alloy in this study. The effects of Sm addition on the microstructures and mechanical properties of the Mg-10Y alloy, especially the aged Mg-10Y alloy, were investigated. The microstructure observation and tensile tests were performed by using an optical microscopy, a scanning electron microscopy and a universal material testing machine, respectively. The phase analysis was performed using X-ray diffractometer. The results show that the 0.5wt.% Sm addition can not only promote the formation of fine and dispersed Mg24Y5 phases, but also improve their morphology and distribution; it also increases the thermal stability of Mg24Y5 phases. Sm addition is seen to increase the ultimate tensile strength of Mg-10Y alloy at elevated temperatures(200, 250, 300 and 350 ℃), while decrease the elongation. But the elongation is still up to 7.5% even at 350 ℃. In the range of 250 ℃ to 300℃, the ultimate tensile strength of the alloy reaches its maximum(with a range average of 235 MPa) and is not sensitive to the temperature change, which is very useful to the application of heat-resistant magnesium alloys. Even at 350 ℃, the ultimate tensile strength of Mg-10Y-0.5Sm is still up to 155 MPa. Considering both of the ultimate tensile strength and elongation, the maximum application temperature of the Mg-10Y-0.5Sm alloy can be up to 300 ℃. The strengthening mechanisms of Mg-10Y-0.5Sm alloy are mainly attributed to dispersion strengthening of Mg24Y5 phase particles with a certain solubility of Sm and grain refinement strengthening of α-Mg matrix. The effects of Sm addition on the microstructures and mechanical properties of the Mg-10Y alloy, especially the aged Mg-10Y alloy, were investigated. The microstructure observation and tensile tests were performed using an optical microscopy, a scanning electron microscopy and a universal material testing machine, respectively. The results of the phase were performed using an X-ray diffractometer. The results show that the 0.5 wt.% Sm addition can not only promote the formation of fine and dispersed Mg 24 Y 5 phases but also improve their morphology and distribution; it also increases the thermal stability of Mg 24 Y 5 phases. Sm addition is seen to increase the ultimate tensile strength of Mg-10Y alloy at elevated But the elongation is still up to 7.5% even at 350 ° C. In the range of 250 ° C to 300 ° C, the ultimate tensil (200, 250, 300 and 350 ° C) e strength of the alloy reaches its maximum (with a range average of 235 MPa) and is not sensitive to the temperature change, which is very useful to the application of heat-resistant magnesium alloys. Even at 350 ° C, the ultimate tensile strength of Mg-10Y-0.5Sm is still up to 155 MPa. Considering both of the ultimate tensile strength and elongation, the maximum application temperature of the Mg-10Y-0.5Sm alloy can be up to 300 ° C. The strengthening mechanisms of Mg-10Y -0.5Sm alloy are mainly attributed to dispersion strengthening of Mg24Y5 phase particles with a certain solubility of Sm and grain refinement strengthening of α-Mg matrix.