The Web-based information systems have been much developed since the Internet is known as a global accessible open network. The Semantic Web vision aims at providing supplementary meaningful information (meta-data) about Web resources in order to facilitate automatic processing by machines and interoperability between different systems. In this paper, we present an approach for the integration of heterogeneous databases in the Semantic Web context using semantic mediation approach based on ontology. The standard OWL language is used here as the ontology description language to formalize ontologies of local data resources and to describe their semantic correspondences in order to construct an integrated information system. We propose an architecture adopting mediator-wrapper approach for a mediation based on OWL. Some illustrations of database wrapping and semantic mediation using OWL are also presented in the paper.