One of the main component's in Gothermal Power Plant steam turbine and therefore a decreased of turbine's performance will effect reliability of the plant to generate power, so plant won't be able reach requested NMC (Net Maximum Capacity) or to be called "de-rating". Decreased turbine performance cause by the main steam content such as H2S, CO2, NH3, CH4, H2, N2, He, Ar, and Ne, that can't be filtered through demister dan separator, for that reason a foulling would be occured in turbine and automatically it will generate less power. Due to effect of the fouling, a process called Turbine Washing System is needed to improve performance of whole power plant. Turbine washing system is mainly needed just to wash the turbine's blades keep perfromance at their best level. The reseacrh is written to analyze how effective turbine washing system to steam turbine's performance and the power has been generated in whole month of march. This analysis is made by using heat balance method, ASME PTC 6, CEI/IEC 953-1, CEI/IEC 60953-3 and X Steam Table Application. Heat balance method used to find the fluid and steam parameters of each part based on eqilibrium of vapor and energy massses in "enthalpy". The result of research, there was decrease in steam chest pressure of 0,025 - 0,05 [barg] perhour and increase load 0,1 - 0,6 [MW] perhour. In addtion steam flow rate will decrease with opening of governor valve at the same opening position before and after the turbine washing. This caused by unmaximized treatment and implication of turbine washing that planned on March for 5 days. Futhermore, use turbine washing will have an effect eroding turbine parts, because water that has been used in this system is untreated condensation water due demaged demin plant. Achievement of turbine washing system can be measured by pressure of steam chest, power generation and openings of governor valve. If there is no difference or changes in steam chest's pressure and flow up to 2 weeks, then this process will be stopped. And if it continous with a longer period of times, there will be erosion on turbines blades