Thin films of barium strontium titanate (Ba0.5Sr0.5TiO3) were fabricated by pulsed-laser deposition technique. X-ray photoelectron spectroscopy and atomic-force microscopy were used to characterize the chemical composition and surfacial profile of the films. The permittivity and dielectric loss as a function of applied electric field were conducted with an alternating current (AC) signal of 50 mV at 100 kHz. The obtained value of dielectric tunability is about 45%. The nonlinear optical properties of the films were determined by a single beam z-scan method at a wavelength of 532 nm with laser pulse duration of 55 ps. The results show that Ba0.5Sr0.5TiO3 thin films exhibit a fast third-order nonlinear optical response with the nonlinear refractive index and nonlinear absorption coefficient of 5.04×10-6 cm2/kW and 3.59×10-6 m/W, respectively.