Ferroelectric (1−x)Na 0.5 Bi 0.5 TiO 3 –xNaTaO 3 (NBT–NTa) thin films, with 0.05<x<0.3, were deposited on a Si/SiO 2 /TiO 2 /Pt substrate using a modified sol–gel method. The dielectric permittivity, the dielectric losses, the voltage-tunability and the ferroelectric behavior of the NBT–NTa thin films were investigated. The thin films with 5mol% and 10mol% NaTaO 3 showed a similar dielectric permittivity (ε≈441 at 1MHz) and voltage-tunability (n r ≈42% at 370kV/cm, 1MHz), whereas the thin films with 30mol% of NaTaO 3 had a lower dielectric permittivity (ε≈370) and voltage-tunability (n r ≈23%). The relaxor-type ferroelectric response in the thin films with 5mol% NaTaO 3 was due to a decrease in the average grain size to below 150nm, resulting in the appearance of single-domain grains.