Ontologies are used to give meaning to resources on the Semantic Web and therefore the Semantic Web?s success in integrating and providing interoperability between different sources depends on the quality of ontologies. In this paper, we propose a set of transformations to improve the quality of ontology. These transformations are based on schema transformation methods that were previously used to improve the quality of database schema. We first define a set of criteria for assessing the quality and then study the positive or negative impact of each transformation on these criteria.