Because of the discrepancies between design parameters and actual plant data, controller tuning is required during the power testing of a new plant. Furthermore, after a certain period of operation, the effects of aging on the sensors and components cause the system performance to change. With the recent improvements in control system hardware, a better control algorithm can be implemented to ensure the safety of the system. Control system tuning and modification are necessary to keep the system at peak performance. The Taiwan Research Reactor (TRR) is a heavy water-type research reactor. During power operation, a large overshoot in neutron power was observed during a change in thermal power demand. Hence, the dynamic performance of the TRR power regulating system was degraded. From the control system point of view, it is worthwhile to determine the cause of the degraded control system and to tune the corresponding controller setting to achieve better performance. In this paper, the performance of the TRR power regulating system is simulated, the course of the large over-shoot in neutron power is identified, and the control system performance is modified. The main cause of the large overshoot in neutron power is the discrepancy in the delay time of the transfer function between neutron power and thermal power through the identification process. The control system is then modified, based on the actual transfer function. Computer simulation and the simplex search method are applied to obtain the new controller settings. In addition, the discrepancy in the delay time of the transfer function provides valuable information for plant maintenance. Although the TRR was closed in 1988, the experience gained will be useful in control system modification for commercial nuclear power plants in the future.