An operator cognitive model has been constructed referring to Rasmussen¡Çs ladder model, and a simulation system of operator performance has been developed. Computer simulation was then performed to investigate subject¡Çs behavior in a cognitive experiment concerning PWR nuclear power plant diagnosis. The knowledge bases used in the simulation were constructed from the interview to the subjects. Confidence on each hypothesis and performance history of operators were compared between the experiment and the simulation, and the simulation could well explain the behavior of the subjects in the results of event identification, confidence on the results, hypotheses conceived during diagnosis, and plant parameters observed, which indicates the validity of the operator model. The order of parameter observation, however, disagreed, and it has been clearly shown that knowledge on system configuration of the plant as well as the physical layout of interface should be considered.