Heavy　metals　as　pollutants　cause　threats　to　the　environment　and　human　health.　Nitrosodimethylamine　(NDMA)　is　a　probable　human　carcinogen.　In　this　research,　the　mechanism　of　NDMA　formation　as　affected　by　heavy　metal　complexes　[MONO](+)　(M　=　Cd,　Pb,　Hg)　was　investigated　using　density　functional　theory　(DFT).　The　results　show　that　the　formation　of　NDMA　from　the　reaction　of　DMA　with　NO2-　can　be　catalyzed　by　the　heavy　metal　complex.　Three　possible　NDMA　formation　pathways　are　obtained.　Moreover,　we　find　that　one　molecule　of　[MONO](+)　(Pathway　1,　M　=　Cd,　Hg;　Pathway　2,　M　=　Cd,　Pb,　Hg)　reacts　directly　with　DMA　via　two　different　pathways,　forming　different　transition　states　to　produce　NDMA.　Another　mechanism　(Pathway　3)　shows　that　two　[CdONO](+)　molecules　will　produce　N2O3　which　reacted　with　DMA　to　give　NDMA.　These　findings　will　expand　our　understanding　of　the　environmental　significance　of　heavy　metal　ions　and　perhaps　help　develop　efficient　methods　to　prevent　the　formation　of　carcinogenic　nitrosamines.