In this masters thesis, a design proces for a combined cycle thermal power plant was elaborated. The plant parameters are based on existing thermal power plant in Croatia. The input data for the start of the design calculation was 80 MWel total power output. The power plant consists of two blocks that can operate independently of each other. Each block consists of a gas turbine part that produces 30 MWel and a steam turbine part that produces an additional 10 MWel. The gas and steam part of the plant are connected by a heat recovery steam generation where the exhaust fumes from the gas turbine deliver heat to the water and the steam which are then used in the steam turbine part of the process. The introduction part describes the operating principle of the gas turbine power plant, the steam turbine power plant and finally the working principle of the combined cycle power plant. Then a short description of the combined cycle thermal power plant located in Croatia "KTE Jertovec" follows. The second part of thesis is based on the analytical thermal calculation of a combined cycle power plant, which consists of technical description of the energy system, the energy balance calculation of the gas turbine part, the energy balance calculation of the steam turbine, the heat recovery steam generator sizing calculation, the general steam generator plan and the layout drawing of the power plant.