In this study, three reaction mechanisms of a benzyne–nickel (Ni) complex ([Ni(C₆H₄)(dcpe)]) with iodomethane during the methylation process were investigated, namely (a) S_N2 reaction of the benzyne–Ni complex with iodomethane, (b) concerted σ-bond metathesis during the bond breaking/forming processes, and (c) oxidative addition of iodomethane to the Ni-center and the subsequent reductive elimination process. DFT calculations revealed that the reaction barrier of the S_N2 reaction is slightly lower than those of the other mechanisms. The results of orbital analyses suggest that [Ni(C₆H₄)(dcpe)] forms a metallacycle structure between benzyne and the Ni^II (3d⁸) center instead of the η²-structure with the Ni⁰ (3d¹⁰) center. The metallacycle structures became inappropriate as the intermediates of oxidative addition in the formation of the Ni^II–Me bond, avoiding further oxidation to the high-valent Ni^IV. The high free energy along σ-bond metathesis was generated from the steric hindrance, thus invoking methylation and Ni–I bond formation concertedly.