Paraelectric BaxSr1−xTiO3 (BST) (x = 0.5 and 0.6) thin films are attractive candidates for the fabrication of various microwave dielectric devices including phase shifters, resonators, filters, oscillators etc. In the present work we have studied the effect of annealing temperature and ambient on the epitaxial quality, surface morphology, dielectric, and phase shifter characteristics of BST thin films deposited on LaAlO3 substrates. The epitaxial quality of the film was markedly improved as the annealing temperature was increased from 1050 to 1100°C. The degree of phase shift increased from 221 to 328° (measured at 14.5 GHz applying a field of 30V/μm) with the improvement of the epitaxial quality of the film. The insertion loss was also increased with the increase in annealing temperature and therefore the effective phase shifter κ factor (defined by the ratio of the degree of phase shift and insertion loss) remained low (∼30°/dB), while annealing these films in N2 ambient significantly reduced the insertion loss, their dielectric breakdown was observed at relatively lower applied voltage as compared to air and oxygen annealed films. The observed electrical behavior was correlated with the composition, chemical state of the constituents and epitaxial quality of the films synthesized under different annealing conditions.