In the paste, different authors [1,2,3,4,5,6,7,8] have developed analytical process models of superplastic sheets bulged into dies of simple shapes such as cylindrical cavities, parallel walls, conical and prismatic dies. An important data for any type of process model is the constitutive equation of superplastic material. In general, the most common method of characterising a material consists in submitting a specimen, with standardised geometry, to a uniaxial tensile test. For superplastic materials the tests were carried out at different strain-rate in order to determine the flow stress as a function of strain-rate. In alternative, to determine the constitutive equation of superplastic materials, biaxial tests was derived from analytical process models [1,4,5,8]. In this study, the method to determine the strain-rate sensitivity coefficient of superplastic materials, based on the analysis of forming process in a prismatic die, was experimentally validated in the laboratory on specimen of material formed at room temperature. The authors have confirmed, by finite element method, the validity of the material characterisation carried out using a prismatic die.