Ontologies play a key role in creating machine-processable Web content to enable the Semantic Web. Extracting domain knowledge from database schemata can profitably support ontology development and then semantic markup of the instance data with the ontologies. The Entity-Relationship (ER) model is an industrial standard for conceptually modeling databases. This paper presents a formal approach and an automated tool for translating ER schemata into Web ontologies in the OWL Web Ontology Language. The tool can firstly read in an XML-coded ER schema produced with ER CASE tools such as PowerDesigner. Following the predefined knowledge-preserving mapping rules from ER schema to OWL DL (a sublanguage of OWL) ontology, it then automatically translates the schema into the ontology in both the abstract syntax and the RDF/XML syntax for OWL DL. Case studies show that the approach is feasible and the tool is efficient, even to large-scale ER schemata.