Polyimide/silicon dioxide nanocomposites were tested for their dielectric strength against nanofiller concentrations between 0% and 14%. The sol-gel process was used for in situ generation of silicon dioxide nanoparticles in a polyamic acid host matrix. Spin-coated and imidized samples with approximately 15 mu m in thickness were then subjected to dielectric breakdown measurements in accordance with ASTM standards. Results showed two distinct regimes of dielectric strength. Higher dielectric withstand capability of nearly 275 kV mm(-1) was exhibited by samples with 0% and 2% silicon dioxide. Higher concentration samples were dielectrically weaker by approximately 45% at 150 kV mm(-1). Broken-down specimens were examined under optical and electron microscopes. An inverse relationship between nanoparticle concentration and breakdown perforation diameter was observed. Hole sizes decreased gradually from 140 to 40 mu m as silicon dioxide content increased from 0% to 6% and ultimately settled near 30 mu m with higher concentrations. The testing results, examined through failure analysis, were explained by breakdown behaviors and mechanisms at different size scales. The findings from this project, in context with previous works and theories, can help establish connections of dielectric strength, perforation diameter, and nanofiller concentration for future polymer nanocomposite research. POLYM. ENG. SCI., 2019. (c) 2019 Society of Plastics Engineers