Thermoforming is characterized by large deformations of the polymeric material being processed. The simulation is complicated by the fact that an elastic material model cannot principally describe the thermoforming deformation precisely. In this respect non-linear and time-dependent viscoelastic behavior of the polymeric material has to be taken into account. In our research we used the K-BKZ model capable of describing viscoelasticity and large deformations. The parameters of this model were determined experimentally using a simple testing method. The method is derived from plug-assisted thermoforming. Computer analysis was used to evaluate the experiments: The tests performed were simulated using FEM (Finite Elements Method) repeatedly with different material parameters. The material parameters leading to a minimum sum of squares of differences between experimental and simulated values estimate the tested material behavior. If has been found that the final shape and the wall thickness distribution show a significant dependence on the material properties in case of deep thermoforming.