Our previous binary turbine system, which employs steam as the primary working fluid, used Rll as the secondary working fluid because Rll has good characteristics as a working fluid in power plants. However, substitute LBMs (Low Boiling temperature Mediums) are being developed because of the ozone layer depletion by CFCs. We investigated the system using R123 in the next stage. Because R123 is also controlled, in the previous paper, modeling of the dynamics of a directly combined binary turbine system using a mixture of R134a and R123 are described. In this paper, the steady state characteristics and dynamic behavior of the system using a mixture of R134a and R123 are investigated taking into account the mole fraction of the mixture. In addition to the investigation using the experimental system, the following are clarified using our model: In the steady-state characteristics, there are remarkable differences in pressure based on the thermodynamic properties and in the load allotment of the two turbines. However, in the dynamic characteristics, the effect of the mole fraction could hardly be observed on the response characteristics of the rotating components to the load change.