The stretching of semi crystalline materials such as isotactic polypropylene leads to an increase of the material characteristics (tensile strength, E-Modulus etc.). The trend towards using wider and faster film production lines causes the need to develop a better understanding of the complex correlation between molecular structure of the raw material, the processing conditions, the morphology of the cast film and the final film properties. In this investigation a viscoelastic model for the simulation of the stretching process is selected. In this study the definition of the suitable material parameters is supported by further characterization of the material. In modern biaxially oriented Polypropylene (BOPP) lines film production speeds of up to 500 m/min can be achieved. The lines have dimensions of up to 30 m (width) times 100 m (length). This means that taking samples out of a running line is difficult and morphological changes cannot be controlled online. The model, which is presented in this paper shows a good compromise between physical understanding and correlation of simulation and experimental data. The model uses simple spring dashpot connections with a physical understanding of the isotactic polypropylene material. With a better understanding of the physical background of the morphological changes and a suitable model of the stretching process it will be possible to improve the product characteristics of BOPP material. Keywords: Films, BOPP, Stretching, Spring Dashpot Model.