This study examined the effect of applied electric field annealing on the microstructure and microwave properties of Ba 0.5 Sr 0.5 TiO 3 (BST) thin films grown on LaAlO 3 substrates. The ultraviolet–visible transmission spectra and micro-Raman spectra of these BST thin films revealed the band-gap and lattice vibration to be strongly dependent on annealing with an applied electric field. Based on the structural analyses and microwave properties, the variation in the band-gap, the chemical binding states, and the tunability of the films were attributed to the combined effects of applied electric field annealing in highly elongated BST thin films.