The dehydrogenation reaction mechanisms of methane catalyzed by a ligated transition metal MH+ (M = Ru, Rh, and Pd) have been investigated theoretically. Activation of methane by MH+ complexes is proposed to proceed in a one-step manner via one transition state: MH+ + CH4 -> MH+CH4 -> [TS] -> (MCH3+)H-2 -> MCH3+ + H-2. Both high-spin and low-spin potential energy surfaces are characterized in detail. Our calculations indicate that the ground-states species have low electron spin and a dominant 4d(n) configuration for RuH+, RhH+, and PdH+, and the whole reaction proceeds on the ground-states potential energy surfaces with a spin-allowed manner. The MH+ (M = Ru, Rh, and Pd) complexes are expected from the general energy profiles of the reaction pathways to efficiently convert methane to metal methyl, thus RuH+, RhH+, and PdH+ are likely to be excellent mediators for the activity of methane. In the reactions of MH+ with methane, the H-2 elimination from the dihydrogen complex is quite facile without barriers. The exothermicities of the reactions are close for Ru, Rh, and Pd: 11.1, 1.2, and 5.2 kcal/mol, respectively.