The binary turbine system that employs steam as the primary working fluid has 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 compared with water) are being developed because of the destruction of the ozone layer by CFCs. In this paper, modeling and experimental investigations into the dynamics of a directly combined binary turbine system using RI23 instead of Rll are described. The system consists mainly of a steam- and an R123-turbine, an R123 vapor generator, and an AC generator. The dynamic behavior of the system is discussed from the viewpoint of the network theory. The components of the system are represented as two- or three-port elements of the network and the vapor flow rate and shaft torque are appropriated as the through variable, and the vapor pressure and rotational speed as the across variable. As a result, a very simple network model based on the dynamics of various turbine systems like this was derived. The validity of the model was proven through comparison with our experimental results, which are the frequency responses examined with respect to generator load changes. In addition, we demonstrate that the mode] is very effective in predicting the dynamic behavior and power generation mechanism of directly combined binary turbine systems.