Self-organized (Ba0.7,Sr0.3) TiO3 (BST) thin film nanocomposites with spherical inclusions have been fabricated. The composite structures were built up from initial amorphous BST films, thanks to an adequate thermal treatment. Microstructural and electrical analyses are reported as a function of crystallization ratio q. Our experimental results agree with recently developed models of composite materials. The main conclusion of the study is that high dielectric performances can be obtained with a half-crystallized material. For a 630 °C annealing temperature (q-ratio equal to 0.47), Vendik commutation quality factor reaches 1500, which is two times greater than a fully crystallized sample annealed at 700 °C.