Knowledge discovery in knowledge base (KDK) is a brand-new task. Its success will directly act on the construction of large knowledge base, and, at present, it is important to the solving of the bottleneck of machine study—discovering knowledge. The main work of this paper is： The inductive structure of KDK based on the facts in knowledge base, and its algorithm and experimental verification; The inductive structure algorithm of KDK for the rules in knowledge base and its experimental verification.