Good-quality polycrystalline Ba0.7Sr0.3TiO3 (BST0.7) thin films were deposited on fused silica substrates and Si substrates having a thick amorphous SiO2 layer at a relatively low temperature of 550 °C by spin-coating metal organic solutions. The thin films were highly transparent to light in the ultraviolet to near-infrared wavelength regions. The optical propagation loss for a 5-μm-wide and 300-nm-thick polycrystalline waveguide based on BST0.7 was about 17 dB/cm at a wavelength of 632.8 nm. An electro-optic Mach-Zehnder interferometer modulator based on the polycrystalline BST0.7 thin film was monolithically integrated on a Si substrate with standard lithography and wet etching. Optical modulation was successfully demonstrated. The estimated electro-optic coefficient (6.7 pm/V) is the highest reported so far for this kind of film deposited on a fused silica substrate or a Si substrate with an amorphous SiO2 layer.