Nanostructured　crystalline　Al/amorphous　AlN　multilayer　films,　with　a　wide　layer　thickness　(h)　range　from　10　to　200　nm,　were　prepared　and　exposed　to　high-temperature　annealing　from　200　to　600　°C.　Microstructure　of　all　the　multilayers　was　highly　stable　under　the　temperature　up　to　400　°C.　Apparent　grain　coarsening　happened　in　multilayers　with　h　＞　~50　nm　under　500　°C.　However,　the　Al　grains　in　h　≤　~50　nm　multilayers　were　extremely　stabilized　under　even　500　°C　annealing,　showing　a　high　thermal　resistance　up　to　about　0.8Tm　(Tm:　melting　point　of　bulk　pure　Al).　The　stabilization　mechanisms　mainly　include　the　constraint　effect　by　heterophase　interface　and　the　nanotwinning　in　Al　layers,　as　demonstrated　by　statistical　measurement　results　and　ex-situ/in-situ　transmission　electron　microscope　observations.　When　the　annealing　temperature　was　raised　up　to　600　°C,　the　heterophase　interfaces　disappeared　in　h　≤　~20　nm　samples　but　were　well　preserved　when　h　＞　~20　nm,　revealing　that　the　thermal　failure　modes　were　quite　sensitive　to　h.　Based　on　the　microstructural　evolution,　a　thermal　failure　map　was　finally　developed　in　variation　with　h　and　annealing　temperature.　Furthermore,　hardness　of　the　multilayers　was　measured　and　hardening　mechanism　was　also　discussed　according　to　the　microstructural　evolution　after　annealing.　©　2020