Ba0.6Sr0.4TiO3 (BST) thin films doped by K (0, 5, 10 and 12.5mol%) (BSTK) were deposited by radio frequency (RF) magnetron sputtering on Pt/Ti/SiO2/Si substrates. Atomic force microscopy and X-ray diffraction analysis were used to investigate the structure and morphology of the BST thin films. The dielectric measurements were conducted on metal-insulator-metal capacitors at the frequencies from 1 kHz to 1 MHz. It was found that the K concentration in BST thin films has a strong influence on the material properties including surface morphology, dielectric, and tunable properties. The surface root-mean-square roughness of the BSTK films decreased when 5mol% K was doped and increased with increasing content of K hereafter. The dielectric constant, dielectric loss and tunability increased when K was doped in the BSTK thin films. The (Ba0.6Sr0.4)(95%)K5%TiO3 thin film exhibited the highest tunability of dielectric constant of about 62.3% and a figure of merit of 26.1 at a maximum applied bias field at 1 MHz. (c) 2007 Elsevier B.V. All rights reserved.