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The major interface between β′-Mg3Sn precipitate plate and the α-Mg matrix in a Mg-9.8wt.% alloy has been investigated using aberration-corrected scanning transmission electron microscopy and first-principles calculations. It is found that Sn atoms orderly distribute in the single layer of the α-Mg matrix immediately adjacent to the broad surface of β′ at the early stage of ageing. These Sn atoms substitute Mg atoms located at the centers of equilateral triangles constituted by three Mg columns in the outmost layer of β′. First-principles calculations suggest that the ordered Sn distribution is energetically favored and it not only decreases the interfacial energy of the β′-matrix interface but also hinders the occurrence of 1/3<011¯0>α shear that thickens the β′ plate.