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The emergence of Web services has led to more interest into Web service composition, which is an active area of research. The formidable problem of efficient and effective composition of existing Web services is the subject of much current attention. The study of workflow model is one of the most important parts and a key layer of Web service composition. The existing approaches have difficulties in modeling dynamic and complex service composition process, so we propose a novel workflow model named Service/Resource Net (SRN) for Web service composition, which is an extended Petri-net-based model with some new elements, such as time, resource taxonomy, condition, etc. To establish formal concept system of Web service taxonomy in SRN, we present Web service semigroup based on group theory. Moreover, meta-service is proposed based on the definition of generating element in cyclic monoid. As the supplement of traditional analysis methods of basic Petri net, some new methods based on graph theory are presented for SRN analyzing and evaluating. For system and methodology validation purpose, SRN is applied to a case study in our research project.