The influence of filler on the crystallization kinetics of isotactic polypropylene is studied on the example of different carbon nanotubes (CNT) in a wide concentration range. Isothermal and non-isothermal crystallization was measured in a temperature range between 0 °C and 130 °C by fast scanning DSC measurements. The acceleration of the crystallization process is described by an efficiency parameter which is derived from the classical crystallization kinetics. This parameter is independent on the filler content and characterizes the ability of the filer to generate additional nuclei. We recommend the efficiency parameter for characterization the influence of fillers and nucleating agents on the crystallization kinetics.