In this paper, we describes the effect of rapid thermal annealing on the electrical properties of (Ba0.5Sr0.5)TiO3 (BST) thin films. BST thin films were grown by the rf magnetron sputtering method with rapid thermal annealing (RTA) at various temperatures. When BST thin films were deposited directly on boron-doped Si(100) (P-type) substrate, silicon-diffused oxide interlayers were formed between BST and P-type Si(100) depending on the RTA temperature. The dielectric constant of BST thin film was improved by RTA and the maximum value (∼100) was obtained at 700° C.