Abstract: Ba_0.5Sr_0.5Nb_zTi_1-zO₃ （BSNT, z =04.12%） thin films were prepared by sol-gel method.The influences of trace element niobium （Nb） on the dielectric properties of the film were studied by means of impedance analyzer HPAgilent 4294A. The dielectric constant （ε_r） is reduced, but dielectric loss （tanδ） is improved, for example, tanδ is reduced from 0.09 to 0.067 at 1 kHz, and Curie temperature T_m moves to lower temperature with the increasing amount of Nb. Better scattering frequency characteristics appear for the thin films with Nb in the range of from 2.0 MHz to 10 MHz. The microstructure of BSNT thin films was studied by XRD, TEM.The crystal structure of the BSNT thin films is tetragonal perovskit,and the grain size is decreased with the Nb content increasing. The ratios of c/a（crystalline cell parameters）of BSNT is changed, and the density is increased.