The stability of the binary combined system of an MHD generator and a synchronous generator, including inverter and power transmission system, is studied numerically. The binary combined power generation system is a pilot plant scale with thermal input of about 110 MW and consists of a MHD generator as a topping cycle and a steam turbine generator as a bottoming cycle. The MHD generator produces a power output of 10 MW, while the synchronous generator produces 30 MW and the total thermal efficiency of the system is beyond 40%. Two systems applying diagonal MHD generators of supersonic and subsonic types are analyzed. The time constant of the synchronous generator is large so that a simplified model is applied to analyze the stability of the system. With the simplified model, the MHD generator is analyzed by a quasi-one-dimensional approximation, the synchronous generator by the voltage behind transient inductance model, the inverter system by the equivalent equation and the transmission system by the four terminal constant model. This study reveals that both types of MHD generators are stable under either steady conditions or a fault occurring in the transmission line. If the a.c. breaker immediately cuts off the faulted line when the faults occur in the transmission line, the effect of the faults on the MHD generator is small and the system is stable.