The electric field tuning property of the 100-kHz dielectric constant of Ba0.5Sr0.5TiO3 (BST) thin films deposited on Pt/Ti/SiO2/Si substrates by a hydroxide-alkoxide-based sol-gel method was investigated by varying the annealing temperature, the thickness and the precursor solution concentration. The tunability and dielectric constant of the BST thin films increased with increasing annealing temperature, indicating the dependence of the tunability on parameters of the film quality, such as crystallinity and grain size. A similar to 235-nm film annealed at 700 degreesC showed tunability of approximately 48%. Variation of the precursor solution concentration resulted in a different microstructure, which in turn affected the tunability of the film. The dielectric tunability of the film showed a small increase as the film thickness increased in contrast, the figure of merit decreased because of the increase in dielectric loss. Experimental results showed that increases in dielectric constant and tunability are always associated with larger dielectric loss, which effectively decrease the figure of merit.