Recently, the first examples of direct vinylation of 1-substituted imidazoles at the 2-position of the imidazole nucleus have been described (J. Org. Chem. 2008, 73, 9155-9157). I-Substituted imidazoles are C(2)-vinylated with 3-phenyl-2-propynenitrile at room temperature without catalyst and solvent to afford 3-(1-organyl-1H-imidazol-2-yl)-3-phenyl-2-propenenitriles, mainly (ca. 95%) as (Z)-isomers, in 56-88% yield. Nevertheless, the stereoselectivity of vinylation, which has been elusive over the past decades, is still a big problem to explain. In this paper, the reaction mechanisms of stereoselective C(2)-vinylation of 1-methylimidazole with 3-phenyl-2-propynenitrile have been investigated using density functional theory (DFT). The geometries of the reactants, transition states, intermediates, and products were optimized at the B3LYP/6-31G(d,p) level. The calculated results reveal that the reaction contains three processes: formation of zwitterion, proton transfer, and ring rearrangement. Four possible reaction channels are shown, including two (E)-isomer channels and two (Z)-isomer channels. One of the (Z)-isomer channels has the lowest energy barrier among all the four channels, with the highest energy barrier for 83.62 kJ/mol, so it occurs more often than the others at room temperature, which is in good agreement with experiment. Further calculations of solvation effects show that the title reaction can be carried out more smoothly in the gas phase.