Based on the model, fuel cell size according to hybrid system is selected. The selection is made by using an optimizing process, in which both thermal-flow parameters and design parameters of the fuel cell were estimated. The optimization is based on maximum efficiency as an objective function and regarded to other hybrid system elements (gas turbine, compressor, heat exchangers, etc.). The optimum hybrid system configuration is presented and commented. The sensitivity analysis of main working parameters is made. Obtained results show that adequate selection of the size, design and working parameters of the SOFC is crucial to obtain ultra-high efficient hybrid system. Additionally, both gas turbine and air compressor should be designed for operation with SOFC. The configuration of installed heat exchanger is essential for obtaining a practical behaviour of the hybrid system.