The new paradigm for distributed computing over the Internet is that of Web services. The goal of Web services is to achieve universal interoperability between applications by using standardized protocols and languages. One of the key ideas of the Web service paradigm is the ability of building complex and value-added service-based applications by composing preexisting services. For a service-based application, in addition to its functional requirements, Quality of service (QoS) requirements are important and deserve a special attention. In this paper, we introduce a discrete-event modeling approach for service-based application. This approach is oriented towards QoS assurance through discrete-event simulation.