Knowledge mining and discovery is a new technology {or decision support syslem beginning in thd middle of 1990 s．In this paper，based on the rough set theory that has been deve]oping verv fasr in recent years—we put forward an algorithm for finding a minimal attribute set of the database betflg n1ined and a]so another algorithm for simplifying the discovered ru]es． We improve the rough set method in such a way that it can generate more appropriate rules．In the end，we give an application in disease diagnosis