The crystallization characteristics of the "-, #-and $-phases of isotactic polypropylene were studied for welldefined and fully characterized polymers with varying amounts of stereo- and regio-defects. The specimens enabled us to study separately the influence of the type of chain defect and the concentration of defects on the parameters of interest. A combined defect fraction (CDF) was introduced to describe arbitrary iPP samples with a varying amount of stereo- and regio-defects and a combination thereof. Crystal growth rates were found to decrease linearly with the defect fraction and were substantially stronger influenced by regio-defects as compared with stereo-defects. The deceleration of the growth rate of the #-phase is higher compared to the "-phase with increasing defect fraction. We also found a critical defect fraction, (Xcrit) for which the growth rates of the "- and #-phases are equal. Analysis of the crystallization was performed using the model of Sanchez and Eby. Results of the analysis are in good agreement with the results found for the samples with a variation in the number of stereo-defects. The excess free energy for incorporating a stereo-defect into the trigonal crystal lattice of the #-phase is lower as compared with the "-phase. The theory correctly predicts the critical defect fraction, for which the growth rate of the "- and #-phase is equal.