Current WSDL-based description specifies only the operation signatures and locations of Web services. It is widely recognized that a richer description is needed. WS-Security, WS-Reliable, WSOL, and service interaction protocols are all typical efforts toward this direction. Based on the cognition that a complete description of service is impossible, this paper proposes a pattern based constraint description approach for Web services. We illustrate the approach with a constraint pattern hierarchy which focuses on the messages exchanged between client and server. And a RDF based constraint description language is proposed, so as to give explicit semantics to the specified constraint descriptions. The application on a web-based auction system shows that our approach is feasible.