<p>Biaxially oriented polypropylene (BOPP) is the prevailing dielectrics for power capacitors in new energy vehicles and smart grids, yet the mechanism underlying the regulation of dielectric breakdown performance by isotactic polypropylene (iPP) resin traits has not been thoroughly clarified. Herein, two commercial capacitor-grade iPP resins (PPA and PPB) were fabricated into cast sheets and BOPP films via continuous extrusion-casting and biaxial stretching processes. Results show that the strongly β-nucleated PPB resin exhibited higher β-crystal content and a refined, uniform crystal structure in cast sheets. After stretching, PPA- and PPB-based BOPP films exhibited nearly identical bulk properties including crystallization behavior, molecular chain orientation and viscoelastic performance; however, the surface roughness of PPB-based films was significantly lower than that of PPA-based films. Moreover, for the same initial cast sheet, the surface roughness of the film decreased with the increase in draw ratio. Fitting analysis revealed a significant linear negative correlation between the dielectric breakdown strength of the films and their surface roughness. The results indicate that the homogenization of surface electric field distribution is the fundamental reason for the enhanced dielectric breakdown strength of BOPP films, and the finite element simulation verifies that low roughness can alleviate local electric field concentration in the film surface layer.</p>