Abstract: In order to represent efficiently large systems, a mechanism for hierarchical composition is needed so that the model may be constructed in a structured manner and composed of simpler units easily comprehensible by the designer at each description level. In this paper we formally define the notion of hierarchy for a Petri net based representation used for modeling embedded systems. We show how small parts of a large system may be transformed by using the concept of hierarchy and the advantages of a transformational approach in the verification of embedded systems. A real-life example illustrates the feasibility of our approach on practical applications.