An industrial gas turbine has the characteristic of decreasing output power on hot summer days while electricity demand peaks. Steam injection in gas turbines has been used for many years to prevent the loss of performance of gas turbines caused by high ambient temperature. Vodoley system is a Closed Steam Injection Gas Turbine Cycle (CSIGTC), which is known as a self-sufficient in steam production. In this study the influence of steam injection in Mashhad Power Plant GE-F5 gas turbine parameters was experimentally investigated. Moreover a computer modelling has been used to study water balance in a CSIGTC system. The results show that although system has additional water treatment process, applying it increases thermal efficiency up to 35.33 percents for 25 MW of power output at an ambient temperature of 40 ˚C. Also water balance considerations show for a direct contact condenser of 96% efficiency and the exhaust temperature of 420 ˚C at turbine outlet, about 7.8 kg/s of make up water is needed under ISO rating of gas turbine (101.3 kPa, 15 ˚C) which must be provided for the CSIGTC system. This amount of water is about 51% of injected steam.