N-Nitrosodimethylamine　(NDMA)　formation　from　tertiary　amines　during　chloramination　has　aroused　widespread　concern　due　to　its　unusually　high　toxicity　and　the　high　NDMA　conversion　yields　of　some　tertiary　amines　especially　for　ranitidine.　However,　the　formation　mechanisms　have　not　been　fully　understood　yet.　In　this　study,　density　functional　theory　(DFT)　calculation　as　a　useful　complementary　approach　to　the　experiments　was　used　to　reinvestigate　NDMA　formation　pathways　from　tertiary　amines　during　chloramination.　The　results　indicate　that　the　formation　of　NDMA　includes　five　processes:　nucleophilic　substitution,　aminyl　radical　generation,　oxidation　of　radical　by　O-2,N-nitroso-compound　cation　formation　and　release　of　NDMA.　In　the　first　two　processes,　the　tertiary　amines　are　more　likely　to　first　react　with　dichloramine　and　the　result　of　which　then　reacts　with　monochloramine　to　generate　aminyl　radicals.　It　is　elucidated　that　dichloramine　plays　a　pronounced　role　in　NDMA　formation.　Moreover,　the　aminyl　radical　formation　accompanied　by　the　release　of　chlorine　radicals　predominates　over　that　by　the　release　of　(NH2)-N-center　dot　radicals　proposed　in　the　literature　mainly　due　to　the　lower　homolytic　bond　dissociation　energy　of　N-Cl　relative　to　that　of　N-H.　Importantly,　the　activation　free　energy　of　the　release　of　NDMA　from　the　N-nitroso-compound　cation,　which　is　fundamentally　related　to　the　previously　reported　heterolytic　ONN(Me)(2)-R+　bond　dissociation　energy,　was　found　to　be　a　key　indication　for　distinguishing　potential　NDMA　precursors　from　tertiary　amines.　The　findings　of　this　work　are　helpful　for　expanding　the　knowledge　of　NDMA　formation　mechanisms　and　predicting　potential　NDMA　precursors　during　disinfection.