In a design process, a designer uses various kinds of design object knowledge, such as physical laws, mechanism library, and manufacturing knowledge. Therefore, it is necessary for a CAD system to handle large scale knowledge bases to support this process. However, it is difficult to use a large scale knowledge base without knowledge about operations of the knowledge itself. This paper proposes a theory of design knowledge operations. First, we analyze the results of design experiments that are records of real design processes. Second, we formalize design knowledge operations based on General Design Theory. Finally we discuss the generality of the proposed formalism of design knowledge operations.