High quality is one of the critical elements contributing to Web service?s success. Monitoring events that are sensitive to quality of Web services is thus an important issue for Web services. This paper proposes an online monitoring approach for web service. This approach is driven by a quality model which covers five kinds of events: the response to client, application execution, changing states of resources, client request, and management operations. We introduce a monitoring framework that collects quality sensitive events by multiple kinds of probes and agents. The framework can do some analysis according to the pre-specified constraints, so as to evaluate the quality of web service. The initial implementation and experiment with a web-based auction example shows that our approach is feasible, and the monitoring cost is affordable.