Highly c-axis oriented Ba0.5Sr0.5TiO3-based composite thin films were grown on MgO (001) single-crystal substrates by pulsed laser deposition and the in-plane dielectric properties of the films evaluated. X-ray diffraction characterization revealed a good crystallinity. The dielectric constant and loss were found to be 200 and 0.001–0.007 at room temperature, respectively. The butterfly-shaped C-V characteristic curve evidenced an enhanced in-plane dielectric tunability of ≫90% in the films at 1 MHz under a dc bias field of 0.8 MV/cm. A brief discussion is given on the abnormal C-V curves. Various tunable microwave applications of Ba0.5Sr0.5TiO3–Bi1.5ZnNb1.5O7 composite thin films are expected.