Creating mappings between database schema and Web ontology is a preconditioning process in the generation of ontological annotations for dynamic Web page contents extracted from the database. In this paper, a practical approach to creating mappings between a relational database schema and an OWL ontology is presented. The approach can automatically construct the mappings by following a set of predefined heuristic rules based on the conceptual correspondences between the schema and the ontology. This automatic mapping is implemented as the core functionality in a prototype tool D2OMapper that has some assistant functions to help the user manually create and maintain the mappings. Case studies show that the proposed approach is effective and the produced mappings can be applied to semantic annotation of database-based, dynamic Web pages.