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Quality of service (QoS) plays an important role in the composition of services. A colored Petri net (CPN) based approach is proposed to model and analyze QoS-driven service composition in a service-oriented architecture (SOA), where all services are considered as dynamic joining and quitting resources. Modeling with an extension of the hierarchical CPN, which is named QSC-net, the QoS-driven features are expressed in the service composition model explicitly, along with the dynamic change of service resources and the uncertain execution of composition processes at runtime. QSC-nets support not only the classical property analysis provided by CPN tools, but also new properties concerning the maximal concurrency of services in the composition model. Compared with existing Petri net based modeling approaches, QSCnets are specially suitable for the modeling and analysis of service composition in a dynamic environment, such as a virtual organization (VO) defined by grid computing.