The nucleation efficiency of carbon nanotubes in a polypropylene (PP) matrix has been investigated by fast scanning calorimetry (FSC). The influence of the nanotubes on the crystallization behavior was studied in isothermal and non-isothermal crystallization experiments. For the latter case the critical cooling rate to ensure a fully amorphous structure was determined as a function of the nanotube content. From this function, the nucleation activity efficiency parameter is derived, which describes the efficiency of nucleation agents independent of temperature and filler content. Isothermal measurements show that the carbon nanotubes accelerate the á-phase crystallization significantly, but do not influence the mesophase formation. The acceleration influence of the nanotubes on the crystallization rate of the á-phase formation crystallization due to nanotubes was found to be related to the (i) number of additional heterogeneous nuclei per nanotube and (ii) the filler influence on the growth rate of the crystalline structure.