The reaction mechanism of Ni(COD)₂ catalyzed hydrodesulfurization of aryl sulfide PhSMe with HSiMe₃ as the reducing agent has been studied by using density functional theory methods. Both PhSMe-coordinated pathway and “ligandless” pathway have been identified and compared. It is found that these two reaction pathways are kinetically competitive and the σ-complex assisted metathesis (σ-CAM) transition state is the highest point on each energy profile for both pathways. Moreover, both the singlet and triplet reaction pathways of ligand substitutions have been compared and found that both singlet and triplet reaction mechanisms are competitive for the ligand substitution of COD with PhSMe on PhSMe-coordinated pathway while the triplet mechanism holds a distinct advantage over singlet one for that of COD with HSiMe₃ on “ligandless” pathway.