An automatic plant start-up system, incorporating a process computer, is in use at Unit No. 1 of Onagawa Nuclear Power Station (a 524-MWe BWR) of Tohoku Electric Power Company. This system, based on and expanded automation techniques proven in fossilfired power plants, is the world's first commercial base system for LWRs. The system automatically performs turbine start-up and controls reactor power by regulating reactor recirculation flow, thus reducing the operator's workload and improving reliability, efficiency, and accuracy of plant operation. Both test data and practical operation have demonstrated the satisfactory performance of this system. This paper discusses system configuration, major functions, and results of performance tests at the factory and installation site, with special emphasis on power control.