Nowadays, SA (software architecture) and CBSD (component-based software development), have shown great advantage in software development. CBSD is a successfully applied down-top methodology to software development; whereas SA still lacks a top-down component composition methodology, especially in process component. Based on workflow and the background of Web application, a component composition method is investigated. Being started with the key elements of business process, such as business object, control flow and data flow, characteristics and interactions between them are formalized. A workflow engine based SA of process component composition is proposed. Except for the control flow, the SA can deal with data flow between business activities, and provide a reference to others in applying archive workflow to component composition.