Mission of this paper was to design and calculate one 100 MW energy block which operate on combined cycle with use of natural gas as fuel. Before calculations, in introduction it's been described how combi cycle came abour and what makes him competitive to single gas and steam turbine systems. The working principle of the process, each of its basic parts, as well as the ways of increasing its efficiency has been described. For the calculation, the selection of gas and steam turbines that meet the required initial requirements and their known working parametets were taken for computation and those that were not are assumed with help of additional literature. The essential thing in the proces was to find parameters of all the characteristic states from which the heat balance was made and ultimately efficiency of entire combined cycle. It was important to show that at the most critical area of the proces, entrance of heated water into evaporator, is a positive temperature gradient, because otherwise it wouldn't work. This is also graphically shown in the t-Q diagram. The conceptual design of this combined working energy block is also drawn and designed in the configuration of 2x1, two gas and one steam turbine.