The Bi1.5MgNb1.5O7 (BMN) thin films were prepared on Pt-coated sapphire substrates by rf magnetron sputter deposition. The as-deposited amorphous films were thermally treated for crystallization and formation of cubic pyrochlore structure. The effects of annealing treatment on microstructures and dielectric properties of BMN thin films have been investigated. X-ray diffraction detected cubic pyrochlore phases in the films annealed at 650 degrees C and above. The grain size and surface roughness of the films increased with annealing temperature. The as-deposited films and the films annealed up to 600 degrees C showed a low value of dielectric permittivity and high value of loss tangent. An abrupt increase in dielectric permittivity and a decrease in loss tangent appeared at 650 degrees C. The thin films annealed above 650 degrees C showed bias voltage dependence of the dielectric permittivity, and the tunability increased with annealing temperature. However, the leakage current density also increased at high annealing temperatures. (C) 2013 Elsevier B. V. All rights reserved.