The number of web-based information systems has been increasing since Internet became the global open network accessible for all. The recent Semantic Web that provides supplementary meaningful information (meta-data) about Web resources facilitates automatic processing of machines and interoperability between different systems. In this paper, we focus on an integration of heterogeneous data sources in the Semantic Web context using a semantic mediation approach based on ontology. We use the ontology description language OWL to formalize ontologies of different resources and to describe their relations and correspondences allowing the semantic interoperability between them. We propose an architecture adopting mediator-wrapper approach for a mediator based on OWL. Some illustrations of semantic mediation using OWL are also presented in the paper.