Ni-rich　LiNixCoyMn1-x-yO2　(NCM)　layered　oxides　are　low-cost　high-energy　density　cathode　materials,　but　plagued　by　its　poor　thermal　stability　incurred　safety　concerns.　The　thermal　failure　process　of　the　layered　cathode　is　accompanied　by　heat　generation　and　oxygen　release,　which　drives　the　battery　into　thermal　runaway　(TR).　Aiming　to　fully　understand　the　TR　process　and　the　structure　evolution,　this　work　applies　diverse　characterization　techniques　onto　a　polycrystalline　Ni-rich　layered　cathode　(LiNi0.83Mn0.05Co0.12O2　(PCN83))　to　comprehensively　investigate　its　thermal　failure　process　at　multiple　scales.　From　macro　level,　we　validate　that　it　is　the　cathode　thermal　failure　that　drives　the　battery　from　the　heat　accumulation　stage　into　TR　in　adiabatic　conditions.　From　micro　level,　transmission　electron　microscopy　(TEM)　verifies　that　the　thermal　failure　of　PCN83　starts　from　150　degrees　C,　which　is　much　lower　than　the　TR　temperature　measured　from　macro　level　tests.　We　reveal　that　the　PCN83　cathode　experiences　sequential　phase　transitions　before　the　TR,　where　the　phase　transition　mechanism　is　illustrated　from　the　atomic　scale　and　the　pore　evolution　process　is　unraveled.　In　situ　heating　TEM　further　reveals　that　thermal　failure　is　preferentially　initiated　from　grain　boundaries　and　defect　regions.　These　findings　provide　an　in-depth　understanding　of　the　whole　thermal　failure　process　of　NMC-based　layered　cathode　materials　and　sheds　new　lights　on　the　rational　design　of　Ni-rich　cathode　materials　with　improved　thermal　safety.