The influence of high amounts of calcium carbonate filler on the crystallization behavior of polypropylene (PP) is investigated by differential scanning calorimetry (DSC) and fast scanning DSC measurements. The non-isothermal crystallization process at industrially relevant cooling rates of about 100 K/s is significantly influenced by the calcium carbonate filler. Isothermal crystallization measurements indicate a new crystallization process in the temperature range between 45 and 80 °C caused by the filler content. To find an explanation for the origin of this process, we have analyzed the interaction between polymer and filler, the crystalline structure, and the crystallization kinetics. From the experimental results, we conclude that the newly observed crystallization process is governed by an additional nucleation process for the growth of α-phase crystals.