<p>The incorporation of β nucleating agents has been demonstrated to improve the toughness of isotactic polypropylene (iPP), but the underlying mechanism remains ambiguous. This study presents a comprehensive investigation on the toughening mechanism in β nucleated iPP through systematic analysis of both macro-properties and micro-structures. The results show that with the addition of 0.1 wt%, three distinct β nucleating agents, BNA-01, zinc adipate (ZnAA) and zinc phthalate (ZnPA), significantly enhances the impact strength of iPP from 3.6&#xa0;kJ/m² to 11.7&#xa0;kJ/m², 6.1&#xa0;kJ/m², and 8.7&#xa0;kJ/m², respectively. To elucidate the toughening mechanism, the relative content of β form iPP (k_β) and crystallinity of nucleated iPP were characterized using wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC). The results reveal a strong correlation between the β form iPP content and toughness, with higher β form iPP content leading to enhanced toughness. However, the results also indicate that when the content of β form iPP is the same, the toughness of nucleated iPP also differs. To address it, the distribution of β form iPP and spherulite size were examined using polarized optical microscopy (POM). The results indicate that, as the content of β form iPP increases, a continuous distribution of β form iPP will be formed, which can significantly improve the toughness of nucleated iPP. Moreover, under the condition of both continuous distributions, smaller spherulite size contributes to a larger fracture surface, consequently demanding more energy for fracture propagation and ultimately enhancing the toughness. Scanning electron microscopy (SEM) was applied to observe the fracture sections of samples to confirm the toughening mechanism. This study provides new insights into the role of β form iPP content, distribution and spherulite size in the toughening of nucleated iPP, offering a deeper understanding of its structure-property relationships.</p>