A total of 77 cases was identified to have human errors from a total of 255 trips occurred from 1978 to 1992 in Korean NPPs. The cases were analyzed to investigate how many human errors occurred on which work conditions to find out the areas of high priority for human error reduction. For the analysis of the 77 trip cases due to human errors, classifications were made for the following four categories ; plant systems, work situation, job types, and error types. Erroneous tasks critically influencing the plant trips were carefully identified and analyzed according to the classifications. Based on the results for the individual cases, the cases were counted for the classification items in each of the four categories, then also for the group of categories to investigate the relationships among the categories in aspects of human error occurrences. As results, the plant systems, work situations, and job types, and error types that are dominant in human errors related to the trips ore identified.