Web services composition is the hotspot research in the field of Web services. Many methods are proposed, however, composition starting from architecture is a new perspective. BPEL4WS is a de facto standard description language for Web services composition. An architecture style of Web services composition system based on BPEL4WS is presented. Then, an architecture description language, WSC/ADL (Web services composition/architecture description language), is designed according to the architecture style. WSC/ADL consists of service components describing Web services, connectors describing the interaction between Web services and configures setting up the relations between instances of service components and connectors, providing the support for top-down development of Web services composition based on architecture. WSC/ADL is discussed in detail with an example and is compared with related work.