The present thesis object is to evaluate the reduction of nitrogen oxides (NOx) emissions through water injection in a heavy duty gas turbine made by EthosEnergy Italia, using Computational Fluid Dynamic (CFD) analysis. The results will be used to improve the current way water injection is performed in the company gas turbines. The purpose is firstly to improve an already existing CFD combustor model in order to get better agreement on NOx emissions, on temperature and on velocity profiles between real measurements and computed ones. Then, the CFD combustor model is modified by a water injection implementation which allow to study the effect of water for NOx emission reduction.