It is widely believed that b-nucleating agent is beneficial for effectively toughening isotactic polypropylene (iPP). However, for the injection molding process, the shearing and thermo-mechanical conditions make the nucleation and crystallization process complicated. In this paper, the effects of injection rate on crystallization of b-nucleated iPP were studied by scanning electron microscope (SEM), two-dimensional wide-angle X-ray diffraction and differential scanning calorimetry (DSC). It is observed that with increasing injection rate, the content of b-crystals exhibits different tendencies in the skin, intermediate layers, and core zone. Specifically, for the intermediate layer, the bcrystals content first increases with increasing injection rate to 85 cm similar to s21, and begins to decrease afterward. By simulating the injection process, the most likely explanation for the b-crystal change is the comparatively high shear rate and low shearing time that the melt experienced. Variations in b-form content are mainly responsible for the mechanical properties of b-nucleated iPP. The results of this study provide a valuable way to control the iPP toughness in the injection molding process. (C) 2016 Society of Plastics Engineers