Carbon Nanotubes (CNT) act as a strong nucleation agent for polypropylene (PP) at low supercooling. The overall crystallization is significantly accelerated. This can be measured easily using conventional DSC. In contrast calcium carbonate (CaCO3) weakly accelerates or even slows the overall crystallization process in this temperature range. We have studied the isothermal overall crystallization kinetics of PP by changing of the concentrations of CNT and CaCO3 significantly in a temperature range from 0 °C to 120 °C. Additionally the non-isothermal crystallization during cooling at cooling rates between 1 K/s and 4000 K/s was measured. The measurements were performed using an ultra-fast scanning calorimeter (Flash DSC 1 from METTLER TOLEDO). From the measurements we can conclude that none of the fillers significantly influence the so-called mesophase crystallization occurring at low temperatures. The high temperature (alpha-phase) crystallization is significantly accelerated by CNT above 50 °C. CaCO3 has no great influence on the crystallization rate of PP at low supercooling. In the medium supercooling rate (around 60 °C), however, the crystallization is significantly accelerated. A new crystallization process occurs. During this process the alpha phase is formed.