MgO-doped Ba0.6Sr0.4TiO3 (BST) thin films were synthesized by rf magnetron sputtering at substrate temperature of 750 °C using single-phase targets with different MgO contents ranging from 0 to 5 mol %. Microstructure, dielectric constant, and leakage current density of the MgO-doped BST films were characterized to understand the influence of the MgO dopant on film properties. Polycrystalline and perovskite phase solid solution films with a dense microstructure were obtained in all deposition conditions. The thickness of the 0,2, and 5 mol % MgO-doped BST films was measured to be 190, 140, and 150 nm, respectively. The electrical and dielectric properties of the BST-containing capacitors are both found to be improved significantly by doping MgO in the BST films. The voltage-dependent capacitance C-V increases with increasing MgO doping up to 5 mol %. The evidence for the asymmetric distribution of charge carriers in the MgO–BST film is derived from the interfacial layer. The leakage current density of the 5 mol % MgO-doped BST capacitors is nearly two orders of magnitude lower and the dielectric constant is about 45% times higher than that of the undoped BST capacitors. Correlation of the material properties with dielectric properties suggests that the 5 mol % MgO-doped BST films are the optimal choice for applications.