The Chemical Gas Turbine (ChGT) system, which has been proposed as a novel system of energy generation, comprises two gas turbines and a steam turbine in a cascade configuration. The combustion in the first gas turbine is performed under a fuel-rich condition, while that in the second one is conducted using the exhaust gas from the first gas turbine as a fuel, because it contains combustible components such as H-2 and CO. In the present study, the thermal efficiency of the ChGT system to which a dimethyl ether (DME) combustion system was applied has been calculated based on thermodynamic analysis to identify the potential of DME as a fuel in the ChGT system. These results are compared with those of CH4 combustion and a conventional combined turbine cycle. In addition, the characteristics of DME combustion, especially under a fuel-rich condition, are investigated empirically.