In recent years a continuously rising demand of short-fiber-reinforced thermoplastics has been noted. The reason for the rising demand are the excellent weight specific mechanical properties compared to non-reinforced thermoplastics. The mechanical properties of the compound of high-strength fibers and the ductile matrix polymer is based on complex physical relations in which the fiber length is a major influencing factor. When processing short fiber reinforced thermoplastics with screw machines (injection molding, extrusion) the fiber length is a resulting material property that is, to some extent, influenced by the process parameters. The extent of the influence and the effects of the process parameters have been carried out in many different studies, mainly with parameter variations. Nevertheless, the influence of the shearing stress on the fiber breaking is also physically modelled. However, the influences of different fiber contents are not taken into account. To investigate the correlation between the fiber content of the thermoplastic polymer and the fiber length degradation a defined flow field is necessary. For this purpose, a test stand with a heated rotating piston and a stationary outer cylinder is used. In the gap between the piston and the outer cylinder the melted material faces shear stress depending on different rotational speeds. For the investigation different short-fiber-reinforced thermoplastics (polypropylene matrix) with different fiber content are used. By measuring the fiber length after different shearing times the time-varying fiber length is determined. With these results the influences of the fiber content and shear stress on the fiber length can be identified.