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 O₂, 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 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.