The liberalization of the electricity power market leads to a stronger competition among electrical utilities. The necessity to reduce pollution due to greenhouse gases, make combined cycle systems one of the best choice to produce energy because of their high efficiency and the use of low carbon content fuels. So, it is of great interest to define a strategy for the optimization of these systems, in order to get greater thermodynamic performance from them. This study presents a thermodynamic optimization of gas turbine combined cycle system. The variable parameters selected for the optimization are the steam turbine efficiency, pinch point and feed water temperature for bottoming cycle (steam turbine cycle), the compressor pressure ratio, turbine inlet temperature and turbine and compressor isentropic efficiencies for topping cycle (gas turbine cycle). The others variables are assumed constant. The thermodynamic expressions are formulated as a function of these decision variables. The optimum design conditions giving maximum combined cycle performance have been determined as final output.