Discovering service-on-demand for large numbers of functionality-similar web services is one of the key issues in service discovery study. To find out the proper service among the functionality-similar web services, a merging cluster algorithm regarding QoS-oriented supply and demand is proposed in this paper. To meet the target, FCM clustering is adopted for agglomerative clustering between the user’s QoS requirement information and the QoS information from Web Service resources. Then, the sequence could be determined by similarity computation with the same clustering. Lastly, the numerical example is presented to illustrate that the service-on-demand can be discovered efficiently and effectively.