The present study was conducted to compare the structure and properties of conventional and so-called "high-crystallinity" (hcr) polypropylene (PP) and to establish characteristic features of the latter that are responsible for its superior thermal and mechanical performance. Moreover, structure-properties relationships of her PP blends with metallocene-catalyzed, linear low-density polyethylene (mLLDPE) were compared with those of conventional PP/mLLDPE blends. In Part 1, relationships between rheological behavior (viscosity and melt density) and thermal (transition temperatures and level of crystallinity) and mechanical properties (impact strength and Young's modulus) were analyzed with reference to composition. The rheological and MDSC tests showed that both types of the blends were miscible at the processing temperatures, whereas immiscible in the solid state and in vicinity of the PP melting point. It was found that the improved mechanical properties and the extraordinary high crystallization temperature of her PP land, correspondingly, her PP/mLLDPE blends) are not due to the assumed high level of crystallinity but due to alteration of internal structure of this polypropylene.