As the development of Service-Oriented Computing (SOC), more and more functional similar services are deployed over the Internet. Nowadays, service selection becomes a crucial issue to make SOC more applicable. Troublesome service will affect the whole SOC application which invokes the service. Therefore, when choosing services, not only the functional attributes but also the Quality of Service (QoS) should also be considered. In this work, we develop a framework to collect the indices of service quality, including runtime and non-runtime indices. A fuzzy partial-ordering approach, which takes both quality indices and their uncertainty into consideration, is proposed to evaluate web services. A series of partial-ordering models has been developed to rank web services according to their quality. Case study shows that the proposed approach is effective for selecting the service having the best quality from the collections of functional similar services.