The main goal of this article is to examine the effect of the choice of the thermodynamic working fluid model and the consideration of blades cooling flow rates on the estimation of a real Gaz turbines plants in term of thermodynamic performances. For this purpose, a comparison between three models has been done: the first takes into account blades cooling and keeps the relationship that modeles the compression and the expansion under their differential forms to follow the progressive variation of the according to the temperature. The second represents variation as a function of the temperature by its average value and without considering the turbine blades' cooling. The last, differs from the previous model only by supposing the as a constant value. Data of three real operating GT plants were used for validation. Results highlighted that the first model provided a realistic results compared to the experimental data, principally in terms of the exhaust gas temperature (EGT) and efficiency of the GT plant, while the two others models were not able to reproduce the real thermodynamic behavior of the GT. In fact, the second model overestimates considerably the EGT and the efficiency, while the third one underestimates those last parameters. Taking into account the blade cooling effect corrects the prediction of the studied parameters during the predesign step of GT's plants configurations, so that NOx productions can be reduced for better efficiencies.