Hybrid cooling systems is used to increase the cooling effectiveness of direct evaporative coolers. In this study, a hybrid cooling system including cooling tower, cooling coil and evaporative cooler have been discussed. The major aim of hybrid systems using is to reduce the energy consumption compare to the other cooling methods. So in this research, optimization of hybrid system was investigated. In order to have an accurate performance prediction of hybrid cooling system, a numerical simulation was performed and the results validated using experimental measurements. Moreover, genetic algorithm is used to determine the optimal design parameters. Minimizing the operation cost including water and electricity costs is considered as the objective function. All design constraints and standards were considered in optimization process. Investigated case study indicates that the optimal design can significantly reduce the operation costs. Optimization results revealed that in optimal case, air mass flow of cooling tower is lower than water mass flow. Also, the velocity of air through evaporative pad was obtained the minimum allowable value and the cross section of cooling tower was obtained the highest possible value according to the constraints. Finally, importance of water and electricity in the optimum system design was investigated due to lack of water in some areas.