Follow the all-pervading of Web Services on consequence of the Quality of Service (QoS) for Web Services becomes one of the most important issues progressively. Network service which is a dynamic system has all sorts of unpredictable factors. Those as we mentioned above are crucial to the QoS of Web Services, such as availability, reliability and security. In the paper the issue of effective service monitoring for the sake of improving the QoS of Web Services is considered. A policy-based web services monitoring system comprising a service monitor control center and a plurality of service monitors is proposed in the paper to achieve the requirements of service requesters, where the service monitor control center collects and monitors the status of service monitors. Policies can be setup in terms of Service Level Agreement (SLA) and states of service monitors. For load balancing, monitoring tasks are assigned to the service monitors in the light of SLA and policies for effectively monitoring QoS of web services. Accordingly, the novel policy-based web services monitoring system is implemented, and its superiority and feasibility are demonstrated.