For Web services, a typical Internetware, a new Web service composition model is given: Requirement driven active Internetware entities aggregating from the aspects of active service, requirement driven and automated aggregating. In this model, service Agents discover requirement actively and aggregate around the requirement automatically. The Agent group aggregated for a given requirement becomes an effective multi-Agent system through a process of mechanism design, negotiation, and collaboration. For making Agents have the ability to discover and understand requirements, a function ontology has been created, which includes function descriptions and function decomposition modes in special domain. For the service Agents collaboration, requirement driven automated mechanism design is put forward, and a collaboration model named barycenter model based on justice principle is also constructed. At last a specification for the final multi-Agent system is given.