Relationship between phase separation and crystallization of polypropylene/poly(propylene-1-octene) in-reactor alloy (iPP/PPOc) were studied using optical microscopy, scanning electron microscopy, and differential scanning calorimetry. Optical microscopy was used to monitor nuclei density and spherulite growth rates, providing complementary information about the effect of liquid-liquid phase separation (LLPS) on crystallization behavior. We found that LLPS process had a retardation effect on crystallization rate and had a dominant effect on the final crystalline morphology of the iPP/PPOc alloy. By simply changing the LLPS time or temperature, we could control the size and the distribution of the elastomer phase that dispersed in the iPP spherulites. The growth rate of the spherulites significantly depended on the degree of LLPS. Higher degree of phase separation reduced nuclei density and the growth rate of spherulites. However, it was helpful to the formation of more perfect spherulites. But surprisingly, there seemed to be little variation of crystallinity between the two quenching procedures (i.e., single quench vs. double quench). Overall, the competition between LLPS and crystallization significantly influenced the structural and morphological development of the iPP/PPOc alloy. By controlling the interplay between LLPS and crystallization of iPP/PPOc alloy, it was possible to control the structure and morphology as needed in applications. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013