Dynamic composition of Web services encounters many challenges including selecting qualified services, composing services dynamically, and assuring quality of services. This article proposes a value model and its representation language, VSDL (Value-based Service Description Language), of Web services, and an architecture of value-added service broker of dynamically composing Web services based on the value model. The value model defines a common model of diverse quality aspects of Web services. We also propose a value metamodel representing relationships among values. The value-added service broker provides value-based service selection, composition and assurance of services. To specify service interfaces with values, the VSDL extends the service interface descriptions of WSDL with the value model. Based on the VSDL, the value-added service broker generates process of composing services and generates WS-BPEL codes for composition. We implemented a prototype of value-added service broker and demonstrated the effusiveness with a case study using three dictionary services.