This paper proposes a service-oriented software reliability model that dynamically evaluates the reliability of Web services. There are two kinds of Web services: atomic services without the structural information and the composite services consisting of atomic services. The model first evaluates the reliability of atomic services based on group testing and majority voting. Group testing is the key technique proposed in this paper to support the service-oriented reliability model. Then, the reliability model evaluates the overall reliability of composite services using an architecture-based model and based on reliabilities of the atomic services, execution scenarios, and operational profiles. The reliability model is dynamic and the reliabilities of the services are evaluated in the actual operational environment. A case study is designed, implemented, and analyzed using the design of experiment technique. The results show the significances of the model and its components.