Externalizing the contents which one learns about instructional material is a fruitful way to facilitate or reinforce his/her learning. However, such knowledge externalization does not always occur spontaneously. Alternatively, it is not so easy for learners because they may make excessive mental efforts which will cause cognitive overload. In this paper, we propose an intelligent assistance in externalizing knowledge structure to be learned with instructional material, which in advance provides learners with part of the externalized structure. This intends to show them what to and how to externalize, and to reduce the externalization load. Considering the context of learning a text describing computer vocabulary, this paper also illustrates the intelligent assistance which is called Diagram Tailoring, and describes an experiment on it. As a result, we have ascertained that Diagram Tailoring contributes to enhancing an understanding of a text. We also demonstrate an intelligent learning system which accomplishes Diagram Tailoring. In this system, learners are encouraged to compose a diagram to visualize the processes of externalizing their knowledge. In order to control their diagramming load, in addition, the system properly presents part of the diagram. The load control is done with the learner models representing their diagramming processes and diagramming capability. Such tailoring technique enables the learners to facilitate or reinforce the retention of their externalized knowledge more effectively.