The tensile strength stability of aging hardened Mg-10Y-1.5Sm (mass fraction, %) alloy was investigated by microstructure analysis and tensile tests. The results show that the microstructure of Mg-10Y-1.5Sm alloy consists of α-Mg matrix and Mg₂₄Y₅ phase and the formation of Sm-containing phase is not observed. With the increase of temperature from 20 _oC to 300 _oC, the tensile strength exceeds 200 MPa and has no obvious change. It is insensitive to temperature and the variation range is less than 10 MPa. The strength stability is superior to that of heat resistant magnesium alloy WE54 developed most successfully. The reason is mainly attributed to the hardness stability of strengthening phase Mg₂₄Y₅ enhanced by dissolved Sm.