Concerns　have　arisen　for　the　formation　of　carcinogenic　nitrosamines　in　postcombustion　carbon　capture　(PCC).　Diethanolamine　(DEA),　as　a　commonly　used　amine　in　PCC,　can　react　with　CO2　to　form　diethanolcarbamic　acid　(DECA).　However,　it　can　also　lead　to　the　formation　of　carcinogenic　N-Nitrosodiethanolamine　(NDELA).　In　this　study,　formation　mechanisms　of　NDELA　in　PCC　were　investigated　by　using　density　functional　theory　(DFT).　The　nitrosation　reaction　of　DEA　by　N2O3　is　an　exothermic　process　and　easy　to　occur　due　to　the　low　energy　barrier　of　7.27　kcal/mol.　The　energy　barrier　of　the　nitrosation　of　DECA　by　N2O3　was　calculated　to　be　48.60　kcal/mol,　which　is　much　higher　than　that　of　the　nitrosation　of　DEA　by　N2O3.　It　indicates　that　the　reaction　is　kinetically　not　easy　to　occur,　and　the　nitrosation　of　DEA　by　N2O3　may　be　the　main　reaction　of　NDELA　formation.　The　results　of　this　study　will　be　helpful　to　understand　nitrosamine　formation　mechanisms　and　find　strategies　to　reduce　nitrosamine　accumulation　during　PCC　processes.