Due to constant changes of business requirements and applications, dynamical reconfiguration of service composition becomes an important and challenging task. Web service composition may save time and expense in software development. In this paper, we introduce the abstract service hierarchy in the OWL-S Ontology Framework for dynamic web service composition. In our approach, the web services are composed at the abstract service level which is a higher level of abstraction than concrete services. The super instance of an abstract service is also introduced to help describe the abstract service. The super instance of each abstract service is attached with an instance pool including all instances of the abstract service to facilitate fail-over and dynamic compositions. An algorithm to populate the concrete services to the instance pool is also provided. We illustrate our approach using a case study.