The interrelation between specific beta-nucleation, thermal history, and molecular weight of isotactic polypropylene (PP) has been investigated by wide-angle X-ray scattering, differential scanning calorimetry, and scanning electron microscopy. Samples with a broad range of molecular weight (M-w), from 240 000 to 1300 000, allowed to examine the effect Or PP Molecular structure oil the nucleation sensitivity. N,N'-Dicyclohexylnaphthalene-2,6-dicarboxamide (NU 100) was introduced in the concentrations of 0, 0.01, and 0.03 wt % as a beta-specific nucleating agent into neat PP. Specimens were then processed via compression molding at various processing temperatures and times. Samples containing 0.01 wt % of NU 100 showed a dramatic decrease of nucleation activity into beta-phase with increasing M-w, processing time, and temperature. This effect was ascribed to a partial solubility of nucleator in PP melt and a competition between heterogeneous beta-nucleation and self alpha-nucleation.