A business process specification (BPS) plays the role of a contracted business scenario in the execution of the B2B business process instances. XML based specification languages, such as BPSS, WS BPEL, etc., are usually adopted for the specifications. However, composing complex XML based specifications are not easy for humans. As an alternative, graphical modeling languages such as UML and BPMN can be employed. This paper presents an UML based modeling method for graphically specifying ebXML BPSS instances (namely, business process specifications). UML diagrams, directly matching with ebXML BPSS, can be reused for understanding and generating ebXML BPSS instances. We discuss the value of the method by developing a business reference library in the area of supply chain management.