In this paper, we present in the first part our proposition for mapping specification and generation of transformation definition in the context of Model Driven Architecture (MDA). In the second part, we present the application of our proposition to Web Services platform. We propose a metamodel for mapping specification and its implementation as a plug-in for Eclipse. Once mappings are specified between two metamodels (e.g. UML and WSDL), transformation definitions are generated automatically using transformation languages such as Atlas Transformation Language (ATL). We have applied this tool to edit mappings between UML and Web Services. Then, we have used this mapping to generate ATL code to achieve transformations from UML into Web Services.