Abstract: It is suggested that the knowledge not only refers to rule knowledge induced from facts, but also refers to case knowledge representing facts. Only by integrating rule knowledge with case knowledge can a domain knowledge be well represented. The object oriented technology can be used to represent the two kinds of knowledge, that is, an object refers to a case, and a category refers to a rule. Both case knowledge and rule knowledge are in the same hierarchical structure of categories. Reasoning approach based on the two kinds of knowledge is also provided.