Nitrosamines　are　a　class　of　carcinogenic,　mutagenic,　and　teratogenic　compounds　generally　produced　from　the　nitrosation　of　amine.　This　paper　investigates　the　mechanism　for　the　formation　of　nitrosodimethylamine　(NDMA)　from　the　nitrosation　of　dimethylamine　(DMA)　by　four　common　nitrosating　agents　(NO(2)(-),　ONOO(-),　N(2)O(3),　and　ONC1)　in　the　absence　and　presence　of　CO(2)　using　the　DFT　method.　New　insights　are　provided　into　the　mechanism,　emphasizing　that　the　interactions　of　CO(2)　with　amine　and　nitrosating　agents　are　both　potentially　important　in　influencing　the　role　of　CO(2)　(catalyst　or　inhibitor).　The　role　of　CO(2)　as　catalyst　or　inhibitor　mainly　depends　on　the　nitrosating　agents　involved.　That　is,　CO(2)　shows　the　catalytic　effect　when　the　weak　nitrosating　agent　NO(2)(-)　or　ONOO(-)　is　involved,　whereas　it　is　an　inhibitor　in　the　nitrosation　induced　by　the　strong　nitrosating　agent　N(2)O(3)　or　ONC1.　To　conclude,　CO(2)　serves　as　a　＂double-edged　sword＂　in　the　nitrosation　of　amine.　The　findings　will　be　helpful　to　expand　our　understanding　of　the　pathophysiological　and　environmental　significance　of　CO(2)　and　to　develop　efficient　methods　to　prevent　the　formation　of　carcinogenic　nitrosamines.