Abstract: Regioselective effects of electron and steric hindrance of catalytically active intermediate HCo(CO)₂L coordinated by phosphine ligands on α-hexene hydroformylation were studied based on density functional theory. Phosphine ligands have strong electron-attracting capacity that raises the stability of HCo(CO)₂L. PPh₃ with large steric hindrance suppresses the coordination of α-hexene with HCo(CO)₂L as well as the secondary reaction of the C=C with the Co–H via the branched chain pathway. The energy barrier for the transition state containing linear chain alkyl Co intermediate is lower about 2.73 kcal/mol than that for the transition state with branched chain alkyl Co intermediate, indicating that linear chain pathway is dominant in the addition reaction. Both the electron and steric hindrance effects of phosphine ligands determines the pathway of addition reaction between the C=C of α-hexene and the Co–H. The linear chain addition is preferable that mainly produces linear chain aldehydes.