Nowadays, the effective and adaptive dynamic Web service composition is a major challenge towards the success of Web services. From the viewpoint of user centric paradigm, existing work has limitation on their agility to create a composed service on the fly according to the desire/need of an end-user at a given time/place. This article presents our approach which consists in providing a comprehensive framework for a user centric WS-mediator which is capable of dynamic service composition. It is based on a composition engine which follows user's specific needs and which yields a composed service through a WS knowledge base. Users can mash up the services at run time with their own logic and have a fully dynamic composition with context adaptation through WS-mediator which is also capable of supporting the semantic web.