To develop high-energy density dielectric materials, hybrid BaTiO3 particles with diblock copolymer shielding layers were fabricated and the dielectric properties of barium titanate-copolymer/polystyrene composites were examined. A significantly large breakdown strength of up to 222 V/μm at high permittivity was obtained. In our approach to formation of a robust polymer shell, polystyrene-block-poly(styrene-co-vinylbenzylchloride) (PS-b-PSVBC) was introduced for the complete wrapping of the BT nanoparticles. The two-step processes to form polymer shielding and the resulting self-assembled layers result in excellent dielectric properties, where a charged layer is buried to minimize the development of the breakdown path and leakage current. Consequently, the maintained high breakdown strength, in spite of the large incorporation of BT particles to give high permittivity, afforded a noticeable energy density value of 9.7 J/cm3.