When querying on a large-scale knowledge base, a major technique of im-proving performance is to preload knowledge to minimize the number of roundtrips to theknowledge base. In this paper, an ontology-based semantic cache is proposed for an agentand ontology-oriented knowledge base (AOKB). In AOKB, an ontology is the collection of re-lationships between a group of knowledge units (agents and/or other sub-ontologies). Whenloading some agent A, its relationships with other knowledge units are examined, and thosewho have a tight semantic tie with A will be preloaded at the same time, including agents andsub-ontologies in the same ontology where A is. The preloaded agents and ontologies are savedat a semantic cache located in the memory. Test results show that up to 50% reduction inrunning time is achieved.