Chloramination　of　drinking　waters　has　been　associated　with　N-nitrosodimethylamine　(NDMA)　formation　as　a　disinfection　byproduct.　NDMA　is　classified　as　a　probable　carcinogen　and　thus　its　formation　during　chloramination　has　recently　become　the　focus　of　considerable　research　interest.　In　this　study,　the　formation　mechanisms　of　NDMA　from　ranitidine　and　trimethylamine　(TMA),　as　models　of　tertiary　amines,　during　chloramination　were　investigated　by　using　density　functional　theory　(DFT).　A　new　four-step　formation　pathway　of　NDMA　was　proposed　involving　nucleophilic　substitution　by　chloramine,　oxidation,　and　dehydration　followed　by　nitrosation.　The　results　suggested　that　nitrosation　reaction　is　the　rate-limiting　step　and　determines　the　NDMA　yield　for　tertiary　amines.　When　45　other　tertiary　amines　were　examined,　the　proposed　mechanism　was　found　to　be　more　applicable　to　aromatic　tertiary　amines,　and　there　may　be　still　some　additional　factors　or　pathways　that　need　to　be　considered　for　aliphatic　tertiary　amines.　The　heterolytic　ONN(Me)(2)-R+　bond　dissociation　energy　to　release　NDMA　and　carbocation　R+　was　found　to　be　a　criterion　for　evaluating　the　reactivity　of　aromatic　tertiary　amines.　A　structure-activity　study　indicates　that　tertiary　amines　with　benzyl,　aromatic　heterocyclic　ring,　and　diene-substituted　methenyl　adjacent　to　the　DMA　moiety　are　potentially　significant　NDMA　precursors.　The　findings　of　this　study　are　helpful　for　understanding　NDMA　formation　mechanism　and　predicting　NDMA　yield　of　a　precursor.