In this article, a new methodology for automated hazard analysis of chemical pla nts is described. The developed system is composed of three knowledge bases: unit knowledge base, organizational knowledge base and material knowledge base, and three hazard analysis algorithms: deviation, malfunction and accident analysis algorithm, The proposed algorithm can perform hazard analysis in two viewpoints and represent all conceivable types of accident using an accident analysis algorithm. In addition, it shows the intermediate steps of accident process, so it produces the analysis result which can give a useful information to hazard assessment. It is successfully demonstrated by applying it to diammonium phosphate manufacturing process.