Research is currently very active in using UML for modeling general software systems [4,5] and e-commerce systems in particular [ 1,2,3]. However, there is still little work on formal use of UML in the development of e- commerce systems. Base on our work in [9,10] about a formal use of UML, this shows how a formal method can be used in the development of an e-commerce system. For this purpose, we shall use a case study of an online ticketing system that is being developed for Macao Secretary of Social Affairs and Culture [7]. We will also show that formal use of UML is necessary for developing e-commerce systems that often require high reliability and performance. For example, the online ticketing system is a real-time system accessed concurrently by clients, and there are some possible faults such as network disconnection and long time idles of clients, safety and liveness properties and fault-tolerance should be considered formally. For the booking function of the ticket system, the following properties should be considered in the design.