The detailed structures and the corresponding formation mechanisms of the well-known Al1oRE2Mn7 phase in the conventional die-cast Mg-4Al-4RE-0.3Mn alloy were thoroughly investigated using transmission electron microscopy (TEM).The results indicate that the Al10RE2Mn7 phase ordinarily contains both normal (11(2)1) twins and orientation twins.Both detailed TEM observations and density functional theory calculations indicate that the Al1oRE2Mn7 phase is transferred from the Al8REMn4 phase following an orientation relationship as [010]Al8REMn4//[(1)101]Al10RE2Mn7 and (101)Al8REMn4//(11(2)0)Al10RE2Mn7.Moreover,forming orientation twins in the Al10RE2Mn7 phase is attributed to the blurry regions at incoherent twin boundaries in the AlsREMn4 phase.Finally,these formed orientation twins result in the (11(2)1)twins in the Al10RE2Mn7 phase.