A simulator which can qualitatively estimate the plant behaviour during abnormal situations is developed. The qualitative modeling illustrated by applying it to a fault diagnosis process system study. The system currently can perform fault diagnosis in a plant consisting of pumps, values. tanks,reactors and streams. No, limitation is placed on the number of equipment units or streams since it is a simple matter to create a new frame and working memory element. This paper describes the QSPE (Qualitative Simulator in Proceess Engineering) for system diagnosis. The simulated results show the applicability this simulator to the solution of problems related to chemical process qualitative dynamics.