Reliability of Web services is an important problem for developing electronic business applications. However, when the number of service requests become very huge it is hard to guarantee the availability of a service. In this paper, a novel approach for reliable Web service provisioning based on mobile agent and resource discovery technologies is proposed. In our approach, new service can be appropriately instantiated on demand, therefore reducing the risk of service being unavailable. Then a dispatching algorithm for resources selection and a multi-phase resource planning algorithm for composite Web services are presented. Experimental results show that our approach is more robust and adaptive in highly dynamic environments.