The ONIOM-molecular dynamics (MD) method, which we recently developed, is applied to one of representative organometallic reactions, cis-(H)(2)Pt(PR3)(2) -> H-2 + Pt(PR3)(2) (R = H, Me, Ph, and t-Bu) to give an insight into the dynamical effects of the environment on the reaction. We adopted the two-layered ONIOM methodology and divided the system into the inner part of cis-(H)(2)Pt(PH3)(2) and the outer part of the others. The inner and outer parts are treated by the quantum mechanics (QM) method at the HF level of theory and the molecular mechanics (MM) method with the MM3 force field, respectively. The ONIOM-MD simulations show that the thermal motion of the outer part increases the magnitude of the energy fluctuations of the inner part and promotes the H-2 elimination reaction. These dynamical environmental effects increase in the order, t-Bu > Ph > Me > H, indicating that the reactivity of cis-(H)(2)Pt(PR3)(2) increases in the same order. These results are also supported by an equation derived from the Arrhenius' equation (Matsubara's equation). The snapshots of the reaction for R = t-Bu clearly indicate the new feature of the H-2 elimination process.