Application of image segmentation to biomedical research is now customary. Due to the existence of a rich heuristic knowledge, many users have no deep experience in this field. It is therefore necessary to integrate knowledge-based techniques with image segmentation operators. The purpose of our expert system is to guide users in image segmentation. Its main functions are: suggest a reasonable overall scheme of processing and recommend appropriate operators and algorithms at each stage. The characteristics of this expert system are presented: (a) interaction with users: through “conversation,” the expert system acquires the informations about a given problem; (b) use of belief values which indicate users' descriptions about the image characteristics. By associating image features with belief values, the system gets the informations about the image appearance and makes inference more effectively; (c) local backtracking strategy, which allows the expert system to repeatedly search for a better solution until a satisfactory result is obtained; (d) integrating with an image analysis package, users can directly execute the operations recommended by the expert system. A practical application of the system is then shown in details. Finally, our opinions in designing such a system are discussed.