Morphology and isothermal growth rates of spherulites for the binary blends consisting of an isotactic polypropylene (i-PP) and an ethylene-l-hexene rubber (EHR) were examined as a function of the crystallization temperature ranging from 388 K to 418 K. In this study, two types of EHR’s were employed : "ethylene rich" EHR and "1-hexene rich" EHR. The blends of i-PP with the EHR of 51 mol % l-hexene are miscible in the molten state, whereas the blends with the EHR of 33 mol % l-hexene are immiscible in the molten state. It is found that the isothermal spherulite growth rate of the miscible i-PP/EHR blends decreases with increasing the EHR fraction, whereas the spherulite growth rate of the immiscible i-PP/EHR blends is independent of the blend composition and is the same as that of the i-PP. Optical microscope observation of the miscible blends crystallized isothermally shows that there are no rubber domains either in the intraspherulitic or in the interspherulitic contact regions. On the other hand, the immiscible i-PP/EHR blends show a phase-separated morphology. Furthermore, the number of tangential lamellae of the miscible i-PP/EHR blends is found to be increased by blending of the EHR, leading to the spherulite with negative birefringence. The sign of birefringence of spherulites is unaffected by the regime transition as well as by the fold surface free energy.