Epitaxial thin films of BaxSr1-xTiO3 (BSTO) are grown on (100) MgO single crystal using pulsed laser deposition (PLD). XRD, SEM, and RHEED 'in situ' are used to investigate microstructural growth phenomena. Microwave dielectric properties are measured by a resonant cavity method. Correlations between film structure and microwave dielectric properties are studied as function of film thickness and oxygen pressure during deposition. The results show that BSTO films are c-axis (001) oriented, with a weak (111) disorientation, which appears with increasing the thickness and oxygen pressure and a direct influence of crystallographic structure and stress state of the film on the microwave dielectric properties is pointed out. Dielectric constant values larger than 1000 are obtained for 900 nm thickness weakly disoriented BSTO films and we show that the insertion of an suboxidized BSTO layer as buffer layer at the substrate/film interface, improves the epitaxy and microwave dielectric properties, leading to dielectric constants larger than 2000. (C) 2003 Elsevier B.V. All rights reserved.