Ba0.5Sr0.5TiO3 thin films were deposited onto (100)-LaAlO3 substrates under various oxygen pressures from 50 to 300 mTorr by pulse laser deposition (PLD). X-ray diffraction investigations indicated that all the films were (001)-epitaxially grown on the substrates; however the epitaxy quality and lattice parameters of the films were changed greatly by the different oxygen pressures (Po-2) during deposition. Lower Po-2 under 150 mTorr made weak epitaxy of films and less tetragonal distortion (c/a) of the unit cell. With increasing Po-2, the films became highly epitaxial and also the c/a ratio increased to 1.002 at Po-2 = 300 mTorr. It was noted that the volume of unit cell was very critical to the epitaxy with LaAlO3 substrates, which acted on the distortion of the unit cells by lattice mismatch force in succession. At frequency of 100 MHz, the film deposited at Po-2 = 200 mTorr, characterized by high single-epitaxy and mild cell distortion, had a large dielectric tunability of 42% (at similar to80 kV cm(-1) bias) and small dielectric loss (tg delta) of 0.008 (at 0 kV cm(-1) bias) and therefor the largest figure of merit (K, tunability/tg delta) of 5250 among all. (C) 2002 Elsevier Science B.V. All rights reserved.