It is well accepted that â-crystals of isotactic polypropylene (iPP) have excellent impact toughness and improved elongation at break. The content of â-crystals is a vital factor tuning the toughness of iPP. Since shear flow field and â-nucleating agent could separately induce â-crystals of iPP in an efficient manner, it was supposed that under the coexistence of these two factors more â-crystals were obtained and could improve the toughness of iPP. As a matter of fact, our work found that the formation of â-crystals had a strong dependency on the interaction of â-nucleating agent and shear flow, rather than being simply synergistically induced by both factors. With the increase of shear intensity from 0 to 30 s-1, the content of â-crystals in â-nucleated iPP was decreased from 0.48 (quiescent crystallization) to 0.30, showing a great depression of â-crystals. It was ascribed to the the competitive growth of á- and â-crystals since the existence of abundant á-nuclei induced by shear flow was in the same order of magnitude of â-nuclei provided by the presence of â-nucleating agent. It was latter found that applying a fixed shear flow (30 s-1) at different temperatures also changed the resultant content of â-crystals. When shear temperature was decreased from 180 to 150 °C, â-crystals were completely depressed. At this moment, á-nuclei overwhelmed â-nuclei, due to the thermodyamic preference for á-crystals. Interestingly, by a relaxation process after shear flow but before cooling to crystallize â-nucleated iPP, â-crystals abundantly revived with the prolonged relaxation time. The key lay in the partially dissolved shear-induced oriented precursors as a result of the relaxation process’s having the ability to generate â-crystals by â-nucleating agent. The understanding of dependency of â-crystals on the interaction of â-nucleating agent and shear flow allowed us to better control the content of â-crystals, eventully manipulate the structures and properties of iPP.