Ontologies are the main component of the semantic Web. Constructing large ontologies typically requires to reuse previously existing ones. Modularity is a key requirement for collaborative ontology engineering and for distributed ontology reuse on the Web. In this paper, we propose an approach to automatically compose ontology modules into a global ontology. Our approach is based on similarity measures computed against concept names, attributes and relationships. We also detail the algorithm allowing to merge the ontology modules into a global ontology.