The crystallization behavior of an isotactic polypropylene/linear low-density polyethylene blend (PP/LLDPE) that is modified with styrene-ethylene/butylene-styrene triblock copolymer (SEES) has been investigated using differential thermoanalysis and polarization microscopy. SEES, which enhances the impact resistance of the blend, has an effect on both the nucleation and crystallization kinetics of the polypropylene component. Nucleation half times, nucleation densities, and spherulite growth rates are influenced by the presence of the copolymer. It is found that SEES, depending on its concentration in the blend, increases the adhesion between matrix and PE domains and acts simultaneously as the matrix reinforcer.