The microstructure, surface morphology, and dielectric properties of Ba0.5Sr0.5TiO3 (BST) thin films prepared by radio-frequency magnetron sputtering at substrate temperatures ranging from 350 to 750° C were characterized. The film deposited at substrate temperature 350° C is amorphous, whereas those deposited at substrate temperatures between 450 and 650° C are a mixture of crystalline and amorphous phases. The amount of crystalline phases increases with the substrate temperature. The phase transformation from amorphous to crystalline occurs at 450° C and the amorphous phase transformed completely into crystalline phase at 750° C. Furthermore, the film deposited at substrate temperature 750° C shows a typical columnar structure with an average size of 100 nm in width extending across the entire film thickness. Dielectric constant of the BST films increases with the substrate temperature due to an increase of the crystalline phases at higher substrate temperature, and reaches a maximum value of 90 at substrate temperature 750° C. Nevertheless, it is also observed that the leakage current of the BST films increases with the content of the crystalline phase. © 2005 The Electrochemical Society. All rights reserved.