The aim of this study is to investigate the effect of different mineral fillers on the foaming behavior of polypropylene. Therefore, several mineral fillers with different particle sizes and aspect ratios were used as nucleating agents. For a better understanding of the influence of the fillers on the foaming process the degassing pressure of CO2 was determined with an optical spectroscopy technique. Additionally, the influence of the mineral fillers on the rheology was examined with a cone plate rheometer. The changes in the degassing pressure due to the different nucleating agents were compared with the foam morphology of the physically foamed samples. The samples were manufactured with a single screw extruder and CO2 as blowing agent and the foam morphology was characterized in terms of mean cell size, cell density and density. The results showed a strong influence of the mineral fillers on the degassing pressure and the cell density increases with increasing degassing pressure.