Using web service composition technology, business demand can be meet quickly and flexibly, formal description and verification of web services and their composition are very important research issues. A colored Petri net(CPN) based model is proposed to formally define and descript Web services and Web services composition, four structures of composition are mainly descript: sequence, choice, concurrent and loop, an example of web services composition modeled by Colored Petri net is proposed, the accuracy of the model is simply analyzed. The result shows that the demand of accuracy of the model is satisfied.