A density functional theory study has been carried out for oxidative addition of the H-CH3 bond to coordinatively unsaturated 14-electron T-shaped M(L-n)(L＇(3-n)) complexes, where M = Ru or Os, and L and L＇ = PH3 or CO. All the stationary points were determined at the B3LYP/LANL2DZ level of theory. It has been found that there should be two competing pathways in those reactions, which can be classified as a a or pi approach. The former was proved to be more favorable with a very low activation energy. A configuration mixing model has been used to develop an explanation for the origin of the barrier heights as well as the reaction enthalpies. Considering the substituent effect and the nature of the metal center, the following conclusions therefore emerge : for a 14-electron T-shaped trans-M(L ")L-2 complex (L " = ligand trans to the incoming methane), a better electron-donating ligand L " (such as PH3 and Cl) with a heavier transition-metal center (third-row) will be a potential model for the oxidative addition of saturated C-H bonds. In addition, transition-metal fragment ML3 containing more electron-releasing alkylphosphines ligands can facilitate the oxidative addition.