Testability is an important quality factor of software, particularly embedded data-flow software such as avionics software. A lack of testability of such software can badly affect test costs and software dependability. Testability analysis can be used to identify parts of software which are difficult for testing. In this paper, we propose the use of the Static Single Assignment (SSA) form to transform source code generated from data-flow designs into a data-flow representation, and then we describe some algorithms to automatically translate the SSA form into a testability model. Thus, some metrics can be applied to the testability model in order to locate the software parts which induce a weakness of the testability.