There are many activities to improve Nuclear Power Plant(NPP) performance and safety such as equipment improvement, redundancy and the like. Since public perception about NPP safety affects nuclear policy of site selection. Recent researches for NPP safety are including not only technical approaches but also social ones. In. this contexture, the purpose of this study is to develope cuassal loop digaram for assessment of organizational and human factors in NPP safety. Through real data from interview with NPP employees we develope causal loop diagram which can explain the relations of cause and effect among variables, and depict social system for organizational and human factors in NPP safety.