You can also use the WebGet attribute to specify the format for request and response messages By default, operations marked with WebGet send response messages formatted as XML data, and the WCF runtime serializes them as POX objects However, you can send response messages in JSON format by specifying the ResponseFormat property of the WebGet attribute with the value WebMessageFormat.Json, as shown here:
[WebGet(Uri Template = ". . .", ResponseFormat = WebMessageFormat.Json) ]
Windows® Communication Foundation 4 Step by Step