Ba0.3Sr0.7TiO3 (BST) thin film composite structures were fabricated by means of a selective epitaxial growth process. The epitaxial growth of BST on SrRuO3 electrode surface was selectively achieved at 450∼485°C using a prepatterned ultra-thin amorphous BST layer that locally prevented crystallization. This self build-up mechanism resulted in a columnar composite structure, where epitaxial and amorphous BST columns are electrically connected in parallel. The epitaxial BST showed a high permittivity and a high tunability, whereas the amorphous BST showed a significantly lower permittivity and a very weak tunable behavior. The effective permittivity of the composite capacitors decreased linearly upon increasing the amorphous BST concentration q, while the tunability stayed fairly unchanged until q reached 70%, which agrees with the theoretical ferroelectric/dielectric parallel composite model. The results also give indirect evidence of a good separation between both BST phases.