In this thesis has been designed combined cycle power plant with a total output of 25 MW e. Gas turbo - generators and steam turbo - generators power plants are connected by a steam generator (heat recovery steam generator) in the exhaust gas from the gas turbine and the gas turbine is powered by natural gas.Combined power plants with a combined cycle processes are increasingly being seen in the world. The development of such energy systems has led to the fact that these are plants with the highest energy power efficiency when it comes to power generation. In this context, the first part describes the brief history of combined power plants, their development and the condition in the world and Croatia.The second part is based on the technical description of the energy system and the entire project of the necessary main parts for combined power plants with power of 25 MW e. Calculation of combined power plant includes the energy balance of the gas plant, the heat balance diagram for combustion of natural gas, the energy balance of the steam energy system and a calculus of heating surface heat recovery steam generator.