Tertiary amines have been demonstrated to be capable of undergoing nitrosative cleavage to produce carcinogenic N-nitrosamines. The reaction mechanism of the nitrosation of trimethylamine (TMA) to produce N-nitrosodimethylamine (NDMA) was investigated at the CBS-QB3 level of theory. The formation of NDMA from TMA was proposed to be initiated by the formation of an iminium ion, Me2N+=CH2. Two different mechanisms (NOH elimination mechanism and oxidation abstraction mechanism) leading to Me2N+=CH2 were investigated, and the oxidation abstraction mechanism was found to be mainly operative. This result indicates that the oxidation abstraction mechanism plays an important role in the nitrosation of both N,N-dialkyl aromatic and tertiary aliphatic airlines. Starting from the iminium ion, two experimentally proposed mechanisms (pathways I and 2) and one new mechanism (pathway 3) were examined. Pathway I proposes that the iminium ion undergoes hydrolysis to give dimethylamine (DMA), which then call be further nitrosated to NDMA; pathway 2 proposes that the iminium ion reacts with NO2- and forms a neutral intermediate, which then collapses to NDMA. In pathway 3, the iminium ion reacts with N2O3 to give NDMA. Calculation results indicate that in aqueous solution pathway I is more feasible than pathways 2 and 3; moreover, the transformation from the iminium ion to NDMA is the rate-determining step. This work will be helpful to elucidate the formation mechanisms of the carcinogenic N-nitrosamines from the nitrosation of tertiary amines.