Ontology classication is the reasoning service that computes all subsumption relationships inferred in an ontology between concept, role, and attribute names in the ontology signature. OWL 2 QL is a tractable prole of OWL 2 for which ontology classication is polynomial in the size of the ontology TBox. However, to date, no ecient methods and implementations specically tailored to OWL 2 QL ontologies have been developed. In this paper, we provide a new algorithm for ontology classication in OWL 2 QL, which is based on the idea of encoding the ontology TBox into a directed graph and reducing core reasoning to computation of the transitive closure of the graph. We have implemented the algorithm in the QuOnto reasoner and extensively evaluated it over very large ontologies. Our experiments show that QuOnto outperforms various popular reasoners in classication of OWL 2 QL ontologies.