A $300kW$ class molten carbonate fuel cell (MCFC)/gas turbine combined compact system has been designed; the system has a 250-cell MCFC stack and a turbocharger generator (TCG) as part of its gas turbine. The 250-cell stack had trouble with a gas leakage; thus, a modified 125-cell stack was refabricated and operated in the system. Using the operation results of the $125-cell+TCG$ system, the thermal efficiency was estimated for the $250-cell+TCG$ system of the original design. The estimated thermal efficiency is 41.0% high heating value (HHV) (45.4% low heating value); the efficiency is 2% lower than the expected value of the original design. The difference of the thermal efficiency between the estimated and expected values of the 250-cell MCFC stack is due to the increase of the internal resistance caused by the stacking procedure. The 125-cell stack was operated for $1700h$ with the TCG and $3200h$ with an external air supply system at an operating current density of $\u223c1500A\u2215m2$; the maximum thermal efficiency of the $250-cell+TCG$ system was estimated (43.0% HHV) at an operating current density of $1500A\u2215m2$. The cell voltage degradation rate was converted to be $0.39%\u22151000h$ at an operating current density of $2000A\u2215m2$. The thermal efficiency, the stack performance, the temperature distribution of the stack, the performance of the TCG, etc., are discussed in detail.