This paper describes the degradation analysis and the performance diagnostics of the gas-turbine (GT) cycle of a combined cycle power plant (CCPP). Three different operating loads, which are 100%, 75%, and 50%, were tested at different ambient conditions, namely, temperature, pressure, and humidity. First, a degradation model to simulate the GT performance with these various operating loads and conditions has been developed. It is then demonstrated how this degradation affects the GT and its components. The degradation analysis has been performed on 2500 readings obtained during 2 years of operation. After applying the load determining criteria, 60 readings were obtained to represent the full load operation and 40 reading points for each part-load operation. The degradation analysis has been carried out on the basis of actual data obtained from a CCPP; this differentiates this study from the others in the same area. Based on the commissioning test performance of the GT cycle, the model has been validated. The results show that the rate of degradation increases dramatically as the load increases. Moreover, the degradation rate also increases with an increase in the ambient air temperature. However, the degradation rate for the various studied parameters, namely, polytropic efficiencies, GT exhaust mass flow rate, and the overall GT efficiency, has been found to decrease with time. The maximum degradation percentage has been estimated to be -1.71% at full load conditions in comparison with -1.33% and -1.16% at 75% and 50% load, respectively.