Quality of Service is a key factor in web service advertising, choosing and runtime monitoring which is multi-faceted, fuzzy and dynamic. Current researches on web service QoS focus on implementation level performance assurance, ignoring domain specific metrics at application level which are also very important to service users. Industry web service standards lack of QoS expressions, and the support for QoS based service choice-making is very limited. An extended web service QoS model is proposed. Web service QoS metrics are evaluated dynamically according to the service context and the overall QoS is evaluated from multiple metrics according to a configurable fuzzy synthetic evaluation system. A QoS requirement description model is defined to express user?s flexible demands on service?s performance. An interactive web service choice-making process is also provided, which includes QoS as a key factor.