This paper reports some experiences that we utilize Petri Net models to validate the reliability of a composite Web service. To enable a validate process, the PNML (Petri Net Markup Language) are introduced to transform a composite service into a Petri Net model that can be further used by measurable processes. The algorithms and corresponding reliable issues have been proposed, such as the coverability tree for detecting safeness, the incidence matrix & state equation for finding reachable issues, and a transitive matrix for addressing deadlock problems. Empirical results suggest that the PNML is essential to facilitate composite services to become computable models. More benefits can be applied by using proper Petri Net based algorithms.