In this work, we investigated the effects of liquid-liquid phase separation (LLPS) on the crystallization kinetics and morphology of isotactic polypropylene/poly (ethylene-co-octene)(iPP/PEOc) in-reactor alloy with polarized optical microscopy (POM), differential scanning calorimeter (DSC), scanning electron microscopy (SEM), wide angle X-ray scattering (WAXS) and small angle X-ray scattering (SAXS) methods. Based on crystallization kinetics analysis by Avrami equation, we found that the overall crystallization rate was almost independent on LLPS time, whereas was strongly dependent on crystallization temperature. However, by combination with POM, we found that the LLPS played two opposite roles on the overall crystallization rate, i.e. the nucleation rate decreased and the spherulite growth rate increased as increasing LLPS time. It is due to the nucleation rate was dominated by fluctuation-assisted nucleation mechanism and the growth rate was dominated by diffusion-controlled growth. Furthermore, the spherulite size and PEOc domain size of iPP/PEOc in-reactor alloy were significantly dependent on LLPS time; however, the crystallinity was almost not dependent on LIPS time. (C) 2012 Elsevier Ltd. All rights reserved.