A new diagnosis system for a boiling water reactor has been developed. In this diagnosis system, a block-monitoring index, obtained through the use of a dynamic model of a nuclear power plant, is used to indicate changes in the operating characteristics of plant equipment. The performance of this diagnosis system using the block-monitoring index was evaluated using actual plant data and was shown to be a feasible method for detecting and identifying anomalies in the plant at an early stage. Diagnosis takes <1 s, which means that the system has on-line and realtime diagnosis capabilities. It is expected that this system will help in improving the safety and availability of nuclear power plants.