Abstract: A performance study was conducted on the Combined Cycle Power Plant (CCPP) which is one of the most important options for replacement and repowering of the available steam power plant. The thermodynamico performance of these plants depends on the ambient temperature varies considerably from one season to another. The object of the current study is to investigate the effects of these variations on the performance of combined cycle power plants. In this study AL-Mussaib thermal power plant, Iraq has been choseno for repowering, for this purpose, AL-Khairat gas turbines are used and the effect of duct burner is investigated. The Repowered Combined Cycle Power Plant (RCCPP) consists of 4 gas turbines and four Heat Recovery Steam Generator (HRSG) with one steam turbine was associated with the unit in this model. The theoretical analysis was made according to both first and second laws of thermodynamic analysis. A generalized computer program was prepared by Fortran 90 for this purpose. The main conclusion drawn from this study were the net power output, thermal and exergy efficiencies of the RCCPP increases as the ambient temperature decreases. The massoflow rate of steamodecreases with the increase of ambient temperature and increases with the increase of compressor pressure ratio and TIT. The exergy destruction in the combustion chamber and HRSG decreases while the exergy destruction of the condenser increases greatly as the ambient temperature increases.