In this presented paper the concept of repowering in existing steam cycle power plant was discussed. Mentioned 300 MW steam cycle power plant is located on 1724 m above sea level and equipped with once-trough steam boiler with supercritical parameters and powered by natural gas. Using commercial software, four different repowering methods has been simulated on that power plant. There were feed water heating repowering method, hot windbox repowering method, parallel hot windbox repowering method and complete repowering method with maximum supplementary firing. Then thermodynamic analysis of power plant model before and after each repowering method has been investigated. To that end three different gas turbines has been used: Siemens V84.2, ABB GT11N2 and Alstom GT13E2 types. To analyze the models, calculations were performed with two stages: 1) calculation and analyzes of thermodynamic parameters as well as carbon dioxide (CO2) emissions of the power plant model in mentioned above cases. 2) Calculations of thermodynamic parameters in values 100, 90, 80, 70, 60 and 50 % of the power plant net cycle power in the different cases to show the advantage of different repowering methods behavior in part loads.