Steam-generator-tube-rupture accident occurred at Prairie Island unit 1 is simulated using the RELAP/5MOD1 code which has been developed as a best-estimate safety analysis code fo light water reactors. The purpose of the simulation is to examine its capacity as a tool of obtaining high-quality and verified data base neede for developing diagnosic techniques of nuclear power plants. The simulation results agrees fairly well with both the plant Energy Research Institute, and it is concluded that the RELAP/5MOD1 code is effective to simulate the overall plant behavior during the accident, although several items remain for future improvement.