The　protective　function　of　the　oxide　scale　and　the　vulnerability　of　the　subsurface　region　play　key　roles　in　determining　the　high-temperature　durability　of　AlCoCrFeNi　high-entropy　alloys　(HEAs),　yet　their　degradation　mechanisms　remain　insufficiently　understood.　This　study　investigates　the　mechanisms　of　oxide　scale　spallation　and　subsurface　mechanical　degradation　in　the　AlCoCrFeNi　HEA　after　high-temperature　oxidation　at　1000　°C.　Severe　oxide　scale　spallation　is　not　solely　caused　by　residual　stress　but　is　closely　linked　to　vacancy　coalescence　at　the　oxide/alloy　interface,　primarily　induced　by　outward　Al　diffusion　and　the　Kirkendall　effect.　A　bilayer　Al2O3　scale　(≈8.4　μm　thick　after　200　h　oxidation)　forms,　consisting　of　equiaxed　and　columnar　grains.　Moreover,　the　subsurface　region　undergoes　a　body-centered　cubic　(BCC)-to-　face-centered　cubic　(FCC)　phase　transformation,　which　reduces　the　diffusion　coefficient　of　Al　(from　4.17　×　10−11　m2　s−11　in　BCC　to　1.8　×　10−11　m2　s−1　in　FCC),　thereby　enhancing　oxidation　resistance.　Meanwhile,　nanomechanical　testing　reveals　an　≈30%　reduction　in　yield　strength　in　the　subsurface　layer,　attributed　to　vacancy-induced　reductions　in　stacking　fault　energy,　which　promote　dislocation　activity　and　plastic　deformation.　This　study　provides　critical　insights　into　the　coupled　effects　of　oxidation,　vacancy　dynamics,　and　phase　transformation　on　the　high-temperature　performance　of　HEAs,　offering　valuable　guidance　for　their　application　in　extreme　environments.　©　2025　Wiley-VCH　GmbH.