Ontologies are used in the integration of information resources by describing the semantics of the information sources with machine understandable terms and definitions. But, creating an ontology is a difficult and time-consuming process, especially in the early stage of extracting key concepts and relations. This paper proposes a method for domain ontology building by extracting ontological knowledge from UML models of existing systems. We compare the UML model elements with the OWL ones and derive transformation rules between the corresponding model elements. Based on these rules, we define an XSLT document which implements the transformation processes. We expect that the proposed method reduce the cost and time for building domain ontologies with the reuse of existing UML models.