Web service composition allows developers to create applications rapidly.But due to the tremendous growth in the number of Web services available,the Web service composition problem is still a challenging research issue.This paper introduces an automatic Web service composition method which considers both services' input/output type compatibility and behavioral constraint compatibility.The services available are translated into a set of Horn clause-like rules.User's input and output requirements are modeled as a set of facts and a goal statement in the Horn clauses respectively.Then Petri net is chosen to model the Horn clause set and T-invariant technique is used to determine the existence of composite services fulfilling the user's input/output requirements. Two algorithms are presented for obtaining the Petri net models of the composite Web services which satisfy not only the user's input/output requirements but also the user's behavioral constraints.