Microstructure and mechanical properties of an extruded Mg-2Dy-0.5Zn(at.%) alloy during isothermal ageing at 180 ℃ were investigated.Microstructure of the as-extruded alloy is mainly composed of α-Mg phase,14H long period stacking order(LPSO) phase and small amounts of(Mg,Zn)_xDy particle phases.During ageing,the 14H LPSO phase forms and develops and its volume fraction increases with increasing ageing time.Tensile test showed that the peak-aged alloy exhibits similar yield and ultimate tensile strengths and elongation to failure at room temperature,100 ℃ and 200 ℃,but excellent elevated temperature strengths at 300 ℃ as compared to the as-extruded and over-aged alloys.The analysis showed that the excellent elevated temperature strengths of the peak-aged alloy are attributed to the LPSO phase strengthening and the grain refinement strengthening,and the role of the LPSO strengthening is related to not only its amount,but also its morphology. Microstructure and mechanical properties of an extruded Mg-2Dy-0.5Zn (at.%) Alloy during isothermal aging at 180 ° C were investigated. Microstructure of the as-extruded alloy is mainly composed of α-Mg phase, 14H long period stacking order LPSO) phase and small amounts of (Mg, Zn) _xDy particle phases. Age aging, the 14H LPSO phase forms and develops and its volume fraction increases with increasing aging time. Sensory test showed that the peak-aged alloy exhibits similar yield and ultimate tensile strengths and elongation to failure at room temperature, 100 ° C and 200 ° C, but excellent elevated temperature strengths at 300 ° C as compared to the as-extruded and over-aged alloys. the analysis showed that the excellent elevated temperature strengths of the peak- aged alloy are attributed to the LPSO phase strengthening and the grain refinement strengthening, and the role of the LPSO strengthening is related to not only its amount, but also its morphology.