Herein, we reported the formation mechanism of hybrid crystalline (cylindrite) in isotactic polypropylene (iPP)/carbon fiber (CF) via pulling a CF within the iPP melt. The alpha-row nuclei layer closely attached to the surface of CF acts as a self-nucleation site, rather than a heterogeneous nucleation one, to grow cylindrites. As a result, the polymorphic feature of iPP/CF cylindrite is significantly influenced by the microstructure of alpha-row nuclei. With decreasing crystallization temperature (T-c), the polymorphic cylindrite changes from pure alpha-form to mixed alpha-/beta-form and to beta-rich form. The main characteristics of this change include: (a) the outlines of alpha-row nuclei layer correspond to wave-like, saw-like, and straight lines; (b) the orientation level of iPP molecules in the alpha-row nuclei layer become higher; (c) the alpha-lamellae rearrange from loose to compact; and (d) the distance between the growth sites of beta-sectors and the surface of CF is evidently longer than in the case of alpha-sectors. Moreover, this study provides a guideline for developing the interfacial enhanced iPP/CF composites through manipulation of polymorphic structure in cylindrites. (c) 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 368-377