The influence of two natural terpene resins on the morphology, phase structure and isothermal crystallization process of iPP was investigated by optical (OM) and electron microscopy (SEM) and differential scanning calorimetry (DSC). It was found that in dependence on temperature, composition and chemical nature of the resin, one, two or three phases can be present. The isothermal spherulite growth rate and overall crystallization rate of the blends are depressed compared to plain iPP. This depression was attributed to the increase of the energies relative to the transport of macromolecules in the melt and to the formation of nuclei of critical size, following the addition of resin. The presence of resin seems to lead to the formation of more regular folding surfaces.