Abstract: In this paper we describe domain-independent concepts in message content in a multi-agent architecture of Intelligent Educational Systems (IES). Although it is well accepted that a common ontology holds the key to fluent communication between agents, most researchers believe that the common ontology is domain ontology. This has been one of the causes of the fact that the work on agent communication has concentrated on general-purpose languages with communication performatives. In IES research, however, we can find task-dependent but domain-independent content, which we could “ontologize”. By domain, we here mean subject domains such as geometry, chemistry, and mathematics, etc. In the multi-agent architecture of IES, we examined the information exchanged between the learner model agent and other agents, and abstracted the domain-independent concepts. We represent them explicitly and domain-independently, to build an ontology. By so doing, we believe that we can move toward a standard for the communication between agents in educational systems, so that the modules/agents can be reused. In this paper we discuss in detail the ontological issues in the message content, and give some examples to show the ontology, that we have designed.