This paper proposes an approach of building OWL2 ontology from data in a relational database. Compared with existing methods, the approach can acquire ontology from a relational database automatically by using a group of learning rules. Approach is independent from database implementation; it is designed based on standards, used to manage database systems. In this paper, we describe object mapping from a relational database and OWL2 ontology including classes, data properties, object properties, instances, axioms and annotations. Axioms are important part of OWL2 specification, and the approach is also suitable for the axiom building process.