Pb0.4Sr0.6MgxTi1-xO3-x (PSMT) thin films were prepared on ITO/glass substrate by a sol-gel technique. It exhibited atypical polycrystalline cubic perovskite structure without any evidence of secondary phase formation. The crystallinity and the dielectric constant of the thin film increased with increasing Mg below x = 0.03 and then decreased with increasing Mg above x = 0.03. The effect of Mg doping content on the dielectric properties of the Pb0.4Sr0.6MgxTi1-xO3-x thin films also depended on the heat-treatment temperature. When Mg doping was light, the highest dielectric constant of the thin film appeared at high heat-treatment temperature. Whereas, when Mg doping was heavy, it appeared at low temperature. The maximum tunabilities of about 30%, 30% and 40% were obtained when Mg doping contents are 0.01, 0.03 and 0.05, respectively. This demonstrates that Mg can improve the microstructure of PST thin film and reduce its lattice distortion. In some conditions, doping Mg could increase the tunable behavior of the thin films. So Mg-doped PST thin films are good candidate for developing tunable devices. (C) 2007 Elsevier B.V. All rights reserved.