Web Services fall under the so-called emerging technologies category and are getting more and more used for Internet applications or business transactions. Since web services are often the foundation of large applications, they need to be reliable and robust. So, we propose in this paper, a robustness testing method of stateful web services, modeled with STS (Symbolic Transition Systems). We analyze the web service observability and the hazard effectiveness in a SOAP environment. Then, we propose a test case generation method based on the two hazards "Using unusual values" and "Replacing /Adding operation names", which are the only ones which can be applied. The Amazon E-commerce web service is taken as example.