With the wide utilization of Petri nets, many researchers suggest that Petri nets should be reused and shared. Emerging the Semantic Web technologies, such as ontologies, can play an important role in this scenario. In this paper, we propose an ontology approach for representing Petri nets. Firstly, we propose a complete formal definition of OWL DL ontologies. Then, Petri nets are introduced and a formal definition of Petri nets is given. On this basis, we propose an approach for representing Petri nets with ontologies, i.e., we translate some key features of Petri nets into classes, properties and axioms of OWL DL ontologies.