In this work, isotactic-polypropylene (iPP) specimens were prepared by a modified injection molding machine, in which high temperature preshear (HTPS) can be imposed on the molten polymer during the plasticizing stage. The effect of HTPS on the microstructure and mechanical property of iPP was investigated. It was found that spherulite size in core region of iPP part decreased steadily with the increasing HTPS duration, indicating that HTPS could substantially enhance iPP nucleation. Moreover, beta-iPP formation correlated strongly with HTPS duration. That is, in the absence of HTPS, beta-iPP existed only in intermediate region; with moderate HTPS duration, beta-iPP could be unexpectedly formed in core region; however, long HTPS duration inhibited beta-iPP formation in both intermediate region and core region. Based on the relationship between beta-iPP formation and HTPS duration, metastable nuclei, instead of alpha-row nuclei, were proposed to be responsible for the development of beta-iPP. Notched Izod impact test showed that moderate HTPS duration enhance the impact strength of injection molded iPP by decreasing the thickness of shear region and elevating beta-iPP crystallinity in core region. Dynamic mechanical test indicated that with the increase of HTPS duration, the storage modulus of injection-molded iPP improves drastically. (C) 2015 Society of Plastics Engineers