Most industrial processes (thermoforming, injection moulding ...) require the understanding of thermo-mechanical behaviour of polymeric sheets. Furthermore, the mastery of the deformation of the polymers becomes an important stake. In order to improve and complete the understanding of the deformation of thermoplastic polymer materials during their forming processes, the problem of modelling the thermoforming process for viscoelastic sheet under large strains is considered. The first step of the process that consists in heating the sheet using infrared lamps is taken into account by including a temperature field in viscoelastic behaviour laws under integral forms. The finite element simulation of the different steps will be presented