Summary: In this study, two different routes were chosen which are expected to influence the elongational viscosity and, thus, the foaming behavior of polypropylene (PP). Electron irradiation of a linear PP was performed at room temperature using different doses. Enhanced long-chain branching was observed with increased irradiation dose. In addition, a linear PP was filled with 5 wt% multiwalled carbon nanotubes (MWNT) by melt mixing at 200 °C. These modified PP materials were investigated with respect to their shear and elongational behavior. After foaming, irradiated PP led to an expansion factor which is about 60% higher than that of the non-irradiated product. In case of PP filled with 5 wt% MWNT the value of the expansion factor is about 70% higher than that of pure PP expanded under the same conditions. The cell structure is much finer in both cases as compared to unmodified PP.