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Several methods and languages have been developed to describe computer system specifications and some of them have been deployed to validate and verify the functionality of the system. No one method is equally appropriate for all application domains. Service oriented architecture and using Web services is a wide growing domain of applications in software engineering these days, so to achieve more goals and benefits in large business works, we need to compose the existing Web services from different organizations. In this paper we discuss about Web service composition mechanisms and related formal methods and verification issues. One of the solutions in highest level is to use Web Service Choreography Definition Language (WS-CDL) to design the composition of services. With regard to this, we need to get sure about the correctness of choreography behavior, before to employ it for developing and executing services collaboration. In this paper we present a method using first order logic notation based on the partial-order planning problems. This method can be used for interactive systems that all participants have a common understanding of interaction rules. This method will be able to model variable specification of the system. It is consist of three parts included precondition, action and effect, so has a simple structure to understand. In this method we can check the properties of the model for reachability of ideal and safe states. We show the method in a Web service interaction case study and verify it in Prolog tool.