A computerized operator aid system for operating procedure monitoring during plant startup from the cold shutdown to the power operation has been developed for 900 MWe pressurized water reactor plants, for which the Kori unit 3 plant has been chosen as the study model. The knowledge base for the system is principally derived from the written form of the standard procedures. The programmed procedures of the system are displayed in Korean on the CRT screen to help operators`` understanding. The main features of the system include the following : 1) computerization of the procedures for the checkup of the initial conditions, the precautions, and the operational procedures, 2) plant status monitoring to detect any plant malfunction or to help operators`` decision making, 3) referring to Technical Specifications on the CRT screen at any time in the process of plant operation, 4) easy prediction of the estimated critical point prior to reactor criticality without complicated hand calculation, 5) easy calculation and evaluation of the shutdown margin, 6) graphical display of the current pressure/temperature in RCS to verify observation of the RCS pressure-temperature limit, and 7) graphical display of axial flux difference during power increase. The results of the system verification indicate that the system performs the intended functions and can be used as an effective tool to minimize operators`` burden during plant startup.