The present article reports the nonisothermal crystallization process and morphological evolution of oriented iPP melt with and without in situ poly(ethylene terephthalate) (PET) microfibrils. The bars of neat iPP and PET/iPP microfibrillar blend were fabricated by shear controlled orientation injection molding (SCORIM), which exhibit the oriented crystalline structure (shish-kebab), especially in the skin layer. The skin layer was annealed at just above its melting temperature (175 degrees C) for a relatively short duration (5 min) to preserve a certain level of oriented iPP molecules. It was found that the existence of ordered clusters (i.e. oriented iPP molecular aggregates) leads to the primary nucleation at higher onset crystallization temperature, and formation of the fibril-like crystalline morphology. However, the overall crystallization rate decreases as a result that the relatively high crystallization temperature restrains the secondary nucleation. With the existence of PET microfibrils, the heterogeneous nucleation distinctly occurs in the unoriented iPP melt and results in the increase of crystallization peak temperature and overall crystallization rate, for the first time, we observed that the onset crystallization temperature has been enhanced further with addition of PET microfibrils in the oriented iPP melt, indicating the synergistic effect of row nucleation and heterogeneous nucleation under quiescent condition. (c) 2008 Elsevier Ltd. All rights reserved.