(Ba0.6Sr0.4)(92.5%) K-7.5% TiO3 (BSTK) thin films doped by Mg from 0 to 6 mol% were fabricated by sol-gel method on a Pt/TiO2/SiO2/Si substrate. The structure and surface morphology of Mg-doped BSTK thin films were investigated as a function of Mg concentration by X-ray diffraction and scanning electron microscopy. The dielectric measurements were conducted on metal-insulator-metal capacitors at the frequency from 100 Hz to 1 MHz and at room temperature. It is found that the Mg concentration in Mg-doped BSTK thin films has a strong influence on the material properties including surface morphology, dielectric and tunable properties. Increasing of Mg content leads to simultaneous decreasing of grain size, dielectric constant, dielectric loss and tenability of Mg-doped BSTK films. The effects of Mg doping on the microstructure, dielectric and tunable properties of Mg-doped BSTK thin films were analyzed. The BSTK thin film exhibited the highest dielectric constant of 1040 and the largest tenability of 73.6%. Mg-doped BSTK thin film (6mol%) showed the smallest dielectric loss of 0.0088 at 1 MHz. Figure of merit (FOM) of Mg-doped BSTK thin films with the optimal Mg content of 4 mol% showed maximum value of approximately 31.21 and its dielectric constant, dielectric loss and tunability were about 336, 0.014 and 44.45%, respectively. These results suggest that improved dielectric properties such as tunability and dielectric loss of 4 mol% Mg-doped BSTK films show the potential in tunable devices. (c) 2006 Elsevier B.V. All rights reserved.