Current semantic trend makes OWL-S be replete with composite services regardless of how the compositions originate, but it provides no way to verify correctness. To alleviate this deficiency, we propose a hierarchical CPNets based model for the specification of process composition so as to raise the composition reliability. The model defines a composite process formulated in OWL-S file in a three-level specification: interface net, orchestration net and composition net, and specifies the control and data flow by mapping control constructs of OWL-S into CPNet formalism. Furthermore, we provide the techniques to analyze and validate the CPNets and explore corresponding meanings of the properties in the context of Web services composition. Based on the model, the behaviors of a composite service can be simulated and validated to allow correcting the composition errors in advance, thus effectively preventing it from runtime failure. Finally we implement an example scenario to illustrate the effectiveness of our approach.