To benefit from distributed architectures, many applications need a coarse grain parallelisation of their programs. In order to help a non-expert parallel programmer to take advantage of this possibility, we have carried out a tool called STEP (Système de Transformation pour l'Exécution Parallèle). From a code decorated with OpenMP directives, this source-to-source transformation tool produces another code based on the message-passing programming model automatically. Thus, the programs of the legacy application can easily and reliably evolve without the burden of restructuring the code so as to insert calls to message passing API primitives. This tool deals with difficulties inherent in coarse grain parallelisation such as inter-procedural analyses and irregular code