The low molecular weight ethylene-propylene random copolymer (LEPR) in a polypropylene (PP)/EPR in-reactor alloy is replaced by high molecular weight EPR (HEPR). It is found that the phase separation rate of PP/HEPR alloy is much slower than that of PP/LEPR alloy. The effect of phase separation on the crystallization kinetics of PP/HEPR alloy is studied and compared with that of PP/LEPR alloy. The pre-phase-separated PP/HEPR alloy exhibits a faster spherulitic growth rate and overall crystallization rate, as compared with the directly quenched one. This is contrary to the PP/LEPR alloy and is interpreted in terms of the relative rate of crystallization and phase separation based on phase diagram. It is also observed that directly quenched PP/HEPR alloy has a larger long period after crystallization. This can be attributed to more inclusion of EPR-rich phase into inter-lamellae, which is advantageous to the mechanical properties.