Ruthenium-catalyzed hydrogenation of CO2 into formic acid was theoretically investigated with the DFT(B3LYP) method, where cis-RuH2(PH3)(4) was adopted as a catalyst model. Theoretical calculations show that (1) CO(2)insertion into the Ru-H bond occurs with an activation energy (E-a) of 29.3 kcal/mol in cis-RuH2(PH3)(4) and with an E-a value of 10.3 kcal/mol in cis-RuH2(PH3)(3); (2) six-membered sigma-bond metathesis of RuH(eta'-OCOH)(PH3)(3)(H-2) occurs with a much smaller E-a value (8.2 kcal/mol) than four-membered sigma-bond metathesis (E-a = 24.8 kcal/mol) and five-membered H-OCOH reductive elimination (E-a = 25.5 kcal/mol; (3) three-membered H-OCOH reductive elimination requires a very much larger E-a value of 43.2 kcal/mol); (4) if Ph-3 dissociates from cis-RuH2(PH3)(4), the CO2 hydrogenation takes place through the CO2 insertion into the Ru-H bond of RuH2(PH3)(3) followed by the six-membered sigma-bond metathesis, where the rate-determining step is the CO2 insertion; and (5) if PH3 does not dissociate from cis-RuH2(PH3)(4) and cis-RuH(eta(1)-OCOH)-(Ph-3)(4), the CO2 hydrogenation proceeds through the CO2 insertion into the Ru-H bond of Cis-RuH2(PH3)(4) followed by the H-OCOH reductive elimination, where the rate-determining step is the CO2 insertion. From the above conclusions, one might predict that (1) excess phosphine suppresses the reaction, (2) the use of solvent that facilitates phosphine dissociation is recommended, and (13) the ruthenium(II) complex with three phosphine ligands is expected to be a good catalyst. The electronic processes and characteristic features of the CO2 insertion reaction and the a-bond metathesis are discussed in detail.