Service Oriented Architecture (SOA) is a computing paradigm which promises to provide transparency to resource access by exposing the available resources as services. SOA is based on service composition for application development or service development. Numerous researches focus on service composition. Each of them tries to solve some particular problems. This paper intends to reify the relevant notions of these models in a Composite Service MetaModel. This metamodel defines all interlaced features and provides a global and explicit vision of the service composition. Moreover, this approach allows the specification of the auto composition process: the composite's ability to dynamically modify its architecture and its composition logics according to the environmental context.