This paper presents a service oriented architecture for testing Web Services. In this architecture, various parties interoperate with each other to complete testing tasks through testing service registration, discovery and invocation. The analysis of the architecture in a typical scenario shows that it has the advantages of supporting dynamic discovery and invocation of testing services as required by the dynamic discovery and invocation of normal functional services without compromising security, privacy and intellectual property rights. It is flexible and extendable. It also helps to reduce the risk of unnecessary disturbances to the normal operations of services due to testing activities. The paper reports a prototype implementation of the architecture by adapting and implementing the ontology of software testing using Semantic Web Services technology. A case study with the WS wrapping of an automated testing tool is also reported, which demonstrated that the architecture is technically feasible.