Many organizations are struggling between the fast delivery of new software and quality assurance. Software testing play a key part in the quality assurance of software systems. Formal testing techniques increase software quality and, at the same time, reduce software development cycle time. This article presents a methodology for the identification and definition of black box test cases based on the functional requirements of a software system. The methodology is applied during the initial phases of software development. The method involves analyzing system requirements and constructing a functional description graph to organize these requirements.