Abstract: Web services composition is widely studied and hotly debated in recent years,and it is one of important components of the Web services architecture.The process model of Web services composition is used to describe the semantic constraints that should be satisfied among service components,and is the basis for automatic service discovery and composition.Taking event algebra as the underlying theory,it is to provide process modeling of Web services composition with a formal approach supporting representation and reasoning of semantics.Aiming at the characteristics and inherent uncertainties implied by services composition itself,it is first extended the classical event algebra and given the definitions,operations and semantics of Web service uncertain event algebra(WSUEA).Further,it is given give the formal WSUEA expressions for sequential,conditional and parallel cases in services composition,as well as the method for computing corresponding probabilities.The feasibility of our methods is verified by experiments.