Geothermal industry unit capital cost and electricity tariff has been competing with lower-cost power generators. This situation has challenged all people that work for the industry to optimize their plant reliability, increase revenue, and reduce cost. Maintenance activities can be considered a critical process which can be very costly if those activities are not managed properly. Part of maintenance management is to determine the optimal maintenance interval with the lowest maintenance cost. This paper determines the optimal maintenance interval of the most critical subsystem in a big-scale geothermal generation facility in Indonesia. The most critical subsystem of the facility is chosen based on reliability value. One of the tools chosen in the industry as a framework for evaluating system reliability is Reliability Block Diagram (RBD). Based on RBD, the most critical sub-system is the Cooling Tower Structure System which consists of two equipment, the Cooling Tower Fan, and the Cooling Tower Structure. The optimum maintenance interval of the Cooling Tower Fan was calculated using the total cost model equation. Sensitivity analysis is also carried out in this paper to determine the cost ratio at which maintenance cost and failure cost calculations must be calculated in detail. To obtain optimum maintenance intervals with a certain confidence interval value, the data resampling with the bootstrap method is applied to the equipment failure data due to the limited amount of data. The difference in maintenance interval value between the original data and the bootstrapped data is relatively small with a maximum of 5.1% difference.