Abstract: With the development of sematic Web, the need of ontology is also growing. However, nowadays most of the data which is stored in relational databases can not be used by sematic Web application. Therefore it is a problem to be solved how to transform the data in relational database to the data that can be used by sematic Web. In consider of these issues, an automatic method of constructing OWL ontologies from relational database is proposed in this paper. The proposed method divides ontology construction process into two parts：semantic finding and ontology mapping. And a graph called M-Graph is defined. With the help of the generating and analysis of the M-Graph combined with some mapping rules, the OWL ontology is automatically constructed. Experiment proves that the proposed method can constructe OWL ontology automatically, and the result ontology is more extensive and accurate.