Undoped and Co-doped Ba0.5Sr0.5TiO3 thin films have been fabricated by pulsed laser deposition to explore low dielectric loss and high tunability materials for tunable microwave devices. The crystal structure and surface morphology have been characterized by X-ray diffraction and atomic force microscopy. The influence of Co-doping on lattice parameter of Ba0.5Sr0.5TiO3 materials has been investigated, and the results show that the variation of lattice parameters with Co concentration is different between targets and thin films. The dielectric loss in the Ba0.5Sr0.5TiO3 thin films with 0.2 and 1 at% Co are 0.064 and 0.039 at frequency of 100 kHz, respectively, which are lower than that of undoped Ba0.5Sr0.5TiO3 thin films. The tunability of permittivity of the Ba0.5Sr0.5TiO3 thin films are 34.6% and 18.7% with 0.2 and 1.0 at% Co-doping at applied electric field 200 kV/cm. (C) 2003 Elsevier B.V. All rights reserved.