The hydrogenation of C7-C9 olefins, which leads to severe loss of octane number in the hydrodesulfurization process of gasoline, was investigated. The reactivity of olefins hydrogenation was affected by the branch group, C=C bond position, and the distance between them. The steric hindrance caused by the branch group was decreased with the increase of the carbon number. However, the effect of the C=C bond position on olefins hydrogenation increased with the increase of carbon number in olefins. Meanwhile, the steric hindrance, caused by the C=C bond position, and the distance between the branch group and C=C bond position can be reduced by isomerization of the C=C bond. Thus, the isomerization of the C=C bond as the first step was the main hydrogenation pathway for internal olefins.