The influence of multi-walled carbon nanotubes (0-5 wt%) on the crystallization process in isotactic polypropylene has been investigated by DSC and fast scanning calorimetry. In the non-isothermal experiments the crystallization temperature shifts to higher temperatures with increasing nanotube content, alpha(CNT), whereas the polymer crystallinity was not significantly influenced. The critical cooling rate, beta(c), at which the PP does not crystallize, increases with increasing of alpha(CNT). From beta(c) and alpha(CNT) a nucleation efficiency parameter was derived, which is independent of crystallization temperature and filler content. Isothermal crystallization experiments allow differentiating between alpha-phase and mesophase crystallization. Nanotubes accelerate only the alpha-phase formation. To describe the efficiency of a nucleation agent an acceleration factor, epsilon, was introduced, which is the ratio of the characteristic crystallization time of unfilled and filled polymer. For the alpha-phase formation the acceleration factor epsilon is related to the number of nuclei per nanotube and the specific effect of the filler on the growth rate. (C) 2017 Elsevier Ltd. All rights reserved.