Achieving good strength-ductility of Mg alloys has always been a crucial issue for the widespread applica-tions of Mg-based structural materials.Herein,an unexpected double-stage strengthening phenomenon was discovered in Mg-8Li-1Y(wt.％) alloys through high pressure (6 GPa) heat treatments over a range of 700-1300 ℃.Attractively,the yield strength values are improved remarkably without losing their duc-tility.The low temperature strengthening mechanism is mainly driven by the formation of large-volume nanoscale contraction twins.In contrast,the high-temperature strengthening reason is ascribed to the presence of densely nano-sized stacking faults.Both coherent interfaces contribute effectively to high mechanical strength without any tradeoff in ductility.