Metastable　structures　of　MgO　overlayer　on　type-III　oxide　surfaces　were　achieved　by　molecular　beam　epitaxy.　Periodic　out-of-plane　dipole　moment　of　substrates　was　revealed　to　play　a　dominant　role　in　the　formation　of　metastable　oxide　structures.　Under　near-equilibrium　epitaxial　conditions,　this　dipole　moment　was　found　to　drive　the　over-grown　MgO　to　inherit　the　out-of-plane　substrate　lattice,　resulting　in　some　new　structures　with　different　stacking　sequences　and　bonding　configurations　from　those　of　thermodynamically　stable　phase.　As　typical　cases,　by　adopting　the　substrate　dipole　moment　along　the　growth　direction,　MgO　overlayers　with　metastable　wurtzite　(WZ)　and　stacking-fault　(SF)　structures　were　formed　on　alpha-Al2O3　(0001)　and　MgAl2O4(111)　surfaces,　respectively.　(c)　2008　Elsevier　B.V.　All　rights　reserved.