A　kind　of　needle-like　sandwich-combined　precipitation　product　(denoted　as　g)　was　found　to　be　formed　concurrently　with　the　equilibrium　beta-phase　in　a　Mg-Gd-Y　alloy　subjected　to　duplex　aging　at　200　degrees　C　and　250　degrees　C,　respectively,　and　its　structure　was　characterized　by　high-angle　annular　dark-field　scanning　transmission　electron　microscopy　(HAADF-STEM)　technique.　It　had　been　clearly　revealed　that　the　eta-precipitate　consists　of　a　new　base-centered　orthogonal　(BCO)　precipitate　as　a　long　core　(B)　which　is　invariably　sandwiched　by　the　face-centered　cubic　(FCC)　Gd(Y)　solute　precipitate　platelets　(F)　as　surface-shells.　The　orientation　relationships　involved　with　the　unique　F-B-F　sandwiched　precipitation　for　the　g　constructing　in　the　Mg　matrix　were　determined　to　be　＜001＞　F//;　＜001＞(B);　{010}(F)//{010}(B)　and　＜100＞(F)//　＜0001＞(Mg),　{010}(F)//{10　(1)　over　bar　0}(Mg),　respectively.　(c)　2020　Elsevier　B.V.　All　rights　reserved.