In the present work, the effect of phase separation on overall isothermal crystallization kinetics of two polypropylene/ethylene-propylene random copolymer (PP/EPR) in-reactor alloys was investigated. It is found that at lower crystallization temperatures (Tc), the overall crystallization rate decreases with increasing phase separation temperature (Ts). This is attributed to the lower linear spherulitic growth rate incurred by the lower PP content in the PP-rich phase at higher Tss. In contrast, at higher Tcs, quenching from a higher Ts to Tc promotes nucleation as a result of more dramatic concentration fluctuation, leading to a faster overall crystallization rate. The overall crystallization rate of the PP/EPR in-reactor alloy prepared by multi-stage sequential polymerization process (MSSP) is retarded by increasing phase separation time (ts). However, prolonging phase separation time has little effect on the crystallization rate of the sample prepared by two-stage polymerization process (TSP). This result can be attributed to the different phase separation rates of these two samples. The SAXS result confirms that at higher Tc, phase separation in the melt before crystallization can retard crystallization, when compared with the directly quenched samples. It is also found that the phase-separated PP/EPR in-reactor alloys exhibit a larger long period because of more amorphous phases included between the lamellar crystals. (C) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013