Heterolytic activation of dihydrogen molecule (H-2) by hydroxo-/sulfido-bridged rutheniumgermanium dinuclear complex [Dmp(Dep)Ge(mu-S)(mu-OH)Ru(PPh3)](+) (1) (Dmp = 2,6-dimesitylphenyl, Dep = 2,6-diethylphenyl) is theoretically investigated with the ONIOM(DFT:MM) method. H-2 approaches 1 to afford an intermediate [Dmp(Dep)(HO)Ge(mu-S)Ru(PPh3)](+)-(H-2) (2). In 2, the RuOH coordinate bond is broken but H2 does not yet coordinate with the Ru center. Then, the H-2 further approaches the Ru center through a transition state TS2-3 to afford a dihydrogen s-complex [Dmp(Dep)(HO)Ge(mu-S)Ru(eta(2)-H-2)(PPh3)](+) (3). Starting from 3, the HH s-bond is cleaved by the Ru and GeOH moieties to form [Dmp(Dep)(H2O)Ge(mu-S)Ru(H)(PPh3)](+) (4). In 4, hydride and H2O coordinate with the Ru and Ge centers, respectively. Electron population changes clearly indicate that this HH s-bond cleavage occurs in a heterolytic manner like H2 activation by hydrogenase. Finally, the H2O dissociates from the Ge center to afford [Dmp(Dep)Ge(mu-S)Ru(H)(PPh3)](+) (PRD). This step is rate-determining. The activation energy of the backward reaction is moderately smaller than that of the forward reaction, which is consistent with the experimental result that PRD reacts with H2O to form 1 and H2. In the Si analogue [Dmp(Dep)Si(mu-S)(mu-OH)Ru(PPh3)](+) (1(Si)), the isomerization of 1Si to 2Si easily occurs with a small activation energy, while the dissociation of H2O from the Si center needs a considerably large activation energy. Based on these computational findings, it is emphasized that the reaction of 1 resembles well that of hydrogenase and the use of Ge in 1 is crucial for this heterolytic HH s-bond activation.