Service discovery is an important aspect of network self-configuration. Solutions to local area service discovery had been put forward and applied to practical systems for several years. In terms of wide area network, the scalability problem of service discovery system must be addressed because there are numberless dynamic services and clients distributed in wide area. To get good scalability, service discovery system should maintain right function and steady performance when the number of services and clients increases continuously. A hash-value based scheme for wide area service discovery is presented in this paper. Based on the model of service description, service query, and match condition, the sets of type name and attributes name in service description and service query are mapped into hash-value and hash-value area respectively. They are used to identify the service directory that will record the service description or satisfy the service query. Each service directory in wide area network is assigned a value area to indicate the range of service descriptions to be recorded in it. All of the service directories are organized into a content addressable tree according to the relationship of their value areas. In such systems, service description is forwarded to certain directory to be recorded in terms of its hash-value, and service query is forwarded to the directories that might satisfy it in terms of its hash-value, area. Simulation results, along with analysis and comparison, show that this scheme can avoid deficiency of the existing schemes and gain favourable scalability.