A number of cycle simulations, which are applied by Molten Carbonate Fuel Cell (MCFC) power plants combined with gas/steam turbines, prove that it is possible to design very highly efficient power plants. However, the stack performance, the operation technology and the performance estimation technology have not yet been established during the initial development stages. The Central Research Institute of Electric Power Industry (CRIEPI) has performed many cell and stack tests and has evaluated the performance under various operating conditions. The operation, performance analysis and estimation methods have been developed for various pressure ranges. Therefore, the accuracy of the plant power estimation has been improved immensely. CRIEPI has also proposed the application of a Li/Na electrolyte instead of a Li/K to achieve higher voltages and a longer stack life. A 10 kW-class short stack consisting of ten 1-m2 cells with a Li/Na electrolyte was operated for more than 10,000 hours, and a very low voltage decay rate was measured during the governmental program. Based on these accomplishments, field tests on small MCFC/GT (gas turbine) hybrid power plants with capacities of several hundred kW will be initiated in Japan throughout the next years.Copyright © 2003 by ASME