A prototype expert system is developed which provides aids for the Emergency Operating Procedures(EOP). This system is mainly intended to reduce information overload and to minimize operator errors in coping with the accident, guiding operations crew into the necessary steps as are expected by the EOP. Implementation to expert system is carried out based on Westinghouse Emergency Response Guidelines (ERG) owing to logical similarities between EOP and ERG. In building expect systems, OPS5 is utilized as an expert system design tool for plant diagnostic purposes. In this work, expert systems for CSFST (Critical Safety Function Status Trees), parts of ORG (Optimal Recovery Guidelines), and FRG (Function Restoration Guidelines) are individually programmed with verification that each program executes properly as intended. Each of modular system is then linked together to form a network of interconnected guidelines. It revealed from this study that the prototype expert system can offer appropriate guidelines to the operators in a clear and concise way under even a stressful situation. Future work will include knowledge extention and validation that the developed system performs generally with desired accuracy with respect to the real environment.