The properties of amorphous BaTiO3thin films deposited on Si substrates were studied as functions of annealing temperature. The film thickness decreased monotonically as the annealing temperature increased up to 600 °C. This reduction is believed to correlate with the densification process observed in the amorphous films during annealing. After the 500 °C annealing, a partial crystallization process was observed. It was also found that the dielectric constant and the index of refraction of the film showed significant changes as the annealing temperature increased from 400 to 500 °C. Correlations between the film density, index of refraction, and the dielectric constant are discussed. A severe reaction at the interface between the film and Si substrate was observed in the 750 °C annealed sample.