The Reactor Management System (RMS), an on-line system with a minicomputer, has been in operation at a boiling water reactor (BWR) nuclear power plant since 1977. The objectives of this system are to perform detailed monitoring and prediction of the core status and to make reactor operation more efficient, simpler, and easier. One of the features of the system is that the RMS utilizes process computer calculated data (such as power distribution and exposure distribution in the core) transferred through the data link, as well as the plant data (such as local power range monitor readings and control rod positions) transferred through the process input/output system. Based on operational experience at the BWR plant, calculation models have been improved to achieve higher accuracy, and new functions have been added to fulfill the operator’s demands. The system has now become a useful tool for the operator. For instance, the power level prediction function has become essential for plant operation at the control rod pattern change.