The hydrogenation rate of styrene in benzene using RhCI(PPh_3)_3 as catalyst increases with the concentration of styrene. But the hydrogenation rate of cyclohexene is rather different, It shows a maximum. So we inquire into the reaction mechanism of cyclohexene hydrogenation catalyzed by RhCI(PPh_3)_3. The rate of hydrogenation was measured at 25±0.15℃, as a function. of catalyst concentration, olefin concentration, triphenylphosphine concentration and hydrogen pressure. The maximum of the reaction rate is interpreted by the formation of RhClL_2S_2. The rate determining step is considered to be the reaction of olefin insertion into one of the Rh-H bonds formed by hydrogenation of RhClL3 to H_2RhClL_3. The hydrogenation rate of the substrate can be described by a third order equation in terms of concentration of H_2RhClL_3. Average error between the results evaluated by this equation and experimental results is about 4.9％. The quantum chemistry calculation gives support to the present mechanism. The hydrogenation rate of styrene in benzene using RhCI (PPh_3) _3 as catalyst increases with the concentration of styrene. But the hydrogenation rate of cyclohexene is rather different, It shows a maximum. So we inquire into the reaction mechanism of cyclohexene hydrogenation catalyzed by RhCI (PPh_3) _3. The rate of hydrogenation was measured at 25 ± 0.15 ° C as a function. Of catalyst concentration, olefin concentration, triphenylphosphine concentration and hydrogen pressure. The maximum of the reaction rate is interpreted by the formation of RhClL_2S_2. The rate determining step is considered to be the reaction of olefin insertion into one of the Rh-H bonds formed by hydrogenation of RhClL3 to H_2RhClL_3. The hydrogenation rate of the substrate can be described by a third order equation in terms of concentration of H_2RhClL_3. Average error between the results evaluated by this equation and experimental results is about 4.9%. The quantum chemistry calculation gives support to the pres ent mechanism.