In this study, we have prepared two beta-nucleated isotactic polypropylene/graphene oxide (beta-iPP/GO) composites with different stereo-defect distribution (named NPP-B and NPP-A). The role of stereo-defect distribution on the crystallization and polymorphic behaviors of samples with varied melt structures were investigated by scanning electron microscopy (SEM), wide-angle X-ray diffraction (WAXD), and differential scanning calorimetry (DSC). It was found that NPP-B (who with more uniform stereo-defect distribution) prefers beta-crystallization; while NPP-A has a strong alpha-nucleation tendency and can hardly form beta-phase; as fusion temperature decreases from 200 degrees C to 168 degrees C, the relative percentages of beta-phase of NPP-B and NPP-A exhibit the same trends, suggesting that the beta-nucleation efficiency is enhanced by the ordered structure effect (OSE) and causes beta-crystallization even in NPP-A. Moreover, the lower limiting temperatures of Region-II of NPP-B and NPP-A are the same but their upper limiting temperatures are different, indicating the uniformity of stereo-defect distribution also influences temperature window for OSE. Related mechanisms were discussed. POLYM. ENG. SCI., 59:1097-1104 2019. (c) 2019 Society of Plastics Engineers