This paper presents a knowledge-based methodology for business process reengineering that uses a case-based reasoning paradigm to provide decision support to its users in the modeling of a current problem and a redesign of critical business processes. As a process modeling tool for representing the business process, the event process chain(EPC) modeling method is used in this paper. We developed a CAPMOSS(CAse-based Process MOdeling Supporting System) to support our proposed methodology. To reengineer a new business process problem, CAPMOSS retrieves from its case base the case that is most similar to the current problem. CAPMOSS uses a retrieved case to guide the structuring of AS-IS models and TO-BE models of a target business process. Using the transformational knowledge of a retrieved case, CAPMOSS helps the user to transform an AS-IS model into a TO-BE model for the target process with ease and the purchasing process in a government R&D institute is explained as an application of this approach.