Aging behavior of Mg-3.6Y-0.5Zr and Mg-2.TNd-0.5Zr alloys was investigated by microhardness measurement and transmission electron microscopy.In the case of Mg-Y-Zr alloy,the presence ofβ″phase,a major strength- ener,having base centered orthorhombic structure with its lattice constants of a_(β″)=0.64 nm,b_(β″)=2.22 nm, and c_(β″)=0.52 nm was identified.In the case of Mg-Nd-Zr alloy aged at 250℃,the presence ofβ″andβ′phases was identified.The crystal structure ofβ″phase was found to be DO_(19) and its orientation relationships with Mg matrix were [0001]_(β″)//[0001]_(Mg) and [01(?)0]_(β″)//[01(?)0]_(Mg).Theβ′phase had face centered cubic structure and its orientation relationships with Mg matrix were [011]_(β′)//[0001]_(Mg) and [(?)1(?)]β′//[(?)110])_(Mg). The Mg-2.TNd-0.5Zr alloy showed higher hardness compared with Mg-3.6Y-0.5Zr alloy. Aging behavior of Mg-3.6Y-0.5Zr and Mg-2.TNd-0.5Zr alloys was investigated by microhardness measurement and transmission electron microscopy. In the case of Mg-Y-Zr alloy, the presence of β “phase, a major strength - ener, having base centered orthorhombic structure with its lattice constants of a - (β ”) = 0.64 nm, b β (β“) = 2.22 nm, and c β (β ”) = 0.52 nm were identified.In the case of Mg-Nd -Zr alloy aged at 250 ° C, the presence ofβ “andβ’phases was identified. The crystal structure of β” phase was found to be DO_ (19) and its orientation relationships with Mg matrix were [0001] _ (β “) // [0001] _ (Mg) and [01 (?) 0] _ (β ”) // [01 (?) 0] _ (Mg) .Theβ’phase had face centered cubic structure and its orientation relationships with Mg matrix were [011] _ (β ’) // [0001] _ (Mg) and [(?) 1? Β’ // [(?) 110]) _ (Mg). The Mg-2.TNd- 0.5Zr alloy showed higher hardness compared with Mg-3.6Y-0.5Zr alloy.