In an earlier context, we have proposed a metadata model for service description. It describes service by three levels of properties: static properties (like its provider, its location and so on), behavior and interface. We have used ontologies in order to index and store the properties provided by this model. We have developed and implement tools which constitute a trader based on ontologies to discover a service by querying its three levels of description from ontologies. Based on the metadata model that we have proposed, we propose in this paper, an approach which allows the client to compose and to combine services by using our trader. This approach defines three models of composition : static, semi_dynamique and dynamique. These models allow the client to benefit from the functionalities of more than one service and to combine them at run-time and at compile-time to get novel services which provide novel functionalities.