The Information Technology system use visualization to represent data in different forms. Some new researches in this field working on extract Knowledge, rapid information retrieval from the graphical diagram. Therefore, data visualization now trends to use ontology approach to build a robust knowledge-based system. The proposed of this paper is to developed ontology approach and use software tools to improve visualized knowledge. Moreover, study data visualization subject in term of ontology and how to facilitate understanding it on the end user. Computer networks ontology adopted as a case study to prove the important of this approach. The Object Role Modeling (ORM) is a visualized notation used to build the prototype for ontology, and OntoGraf module in the Protégé tool used to build ontology.