Well-crystallized Ba0.5Sr0.5TiO3 thin films with good surface morphology were prepared on MgO(1 0 0) substrates by pulsed laser deposition technique at a deposition temperature of 800 degrees C under the oxygen pressure of 2 x 10(-3) Pa. X-ray diffraction and atomic force microscopy were used to characterize the films. The full width at half maximum of the (0 0 2) Ba0.5Sr0.5TiO3 rocking curve and the root-mean-square surface roughness within the 5 mu m x 5 mu m area were 0.542 degrees and 0.555 nm, respectively. The nonlinear optical properties of the films were determined by a single beam Z-scan method at a wavelength of 532 run with laser 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 being n(2) = 5.04 x 10(-6) cm(2)/kW and beta =3.59 x 10(-6) (m/W), respectively. (c) 2005 Published by Elsevier B.V.