Polystyrene-capped barium titanate (BaTiO₃) nanoparticles with sizes of 11 nm and 27 nm were prepared using amphiphilic star-like diblock copolymer templates. The crystal structure evolution of these nanoparticles over a wide temperature range (10–428 K) was investigated by powder X-ray diffraction. The Rietveld refinement indicates that the abrupt structural transitions observed in micron-sized powders become broad as the particle size is reduced to a few tens of nanometers. The orthorhombic phase (Amm2) is observed in the range of 10–388 K, coexisting with the rhombohedral phase (R3c) at lower temperatures and with the tetragonal phase (P4mm) at higher temperatures. At room temperature (300 K), polystyrene-capped BaTiO₃ nanoparticles, both 11 and 27 nm sizes, primarily adopt the tetragonal phase, transforming to the cubic phase (Pmm) at 398 K during heating. The phase evolution of the nanoparticles correlates well with their dielectric behavior. With the Landauer–Bruggeman effective approximation, the dielectric properties at room temperature of the BaTiO₃ core were calculated and the results are in agreement with the size effect of BaTiO₃ nanocrystals.