The Start-up transformers or on-site stand-by transformers are installed for the start-up and shut-down of generator and emergency status in unclear power plant. The on-site power supply configuration using these transformers must be detemined by considering configuration requirements, site characteristics, reliability and availability severely because it is remarkably important for safety and benefit of plant. The start-up or stand-by power supply configuration has been determined considering only safety requirements and construction cost up to now in Korea. I study various reliability estimating methods for the prediction of availability, and estimate the unavailability for the start-up power supply system of two 1,000㎿ unclear power plants. I also detemine the reliability and unavailability and unavailavility of equipment, system and configuration using FTA method.