Polymer nanocomposites are a promising substitute for energy-storage dielectric materials in pulsed power systems. A barium titanate/polyvinylidenefluoride (BT/PVDF) nanocomposite is one of the most widely studied composite systems due to its comprehensive excellent dielectric properties. As the dielectric response of nanocomposites depends strongly on the size of the fillers, in this study, BT/PVDF nanocomposites with 92.3 nm, 17.8 nm and 5.9 nm BT particle fillers are fabricated to reveal the particle size effect of the fillers on the energy storage performance of the polymer nanocomposites. Owing to the small particle size and good dispersibility of the nanofillers, the nanocomposites with smaller BT particles show more uniform and denser microstructures. Moreover, with the increase of the filler fraction, the dielectric results indicate a breakdown strength enhancement in the nanocomposites with sub-20 nm BT fillers, which is quite different from the nanocomposites with normal fillers, and therefore leads to superior energy storage performance. This study provides experimental evidence for the application of ultrafine nanofillers in the nanocomposite for future energy storage systems.