In this paper the dielectric properties of crack-free, Bi12SiO20 thin films were investigated. The films were prepared on Pt/TiO2/SiO2/Si and corundum substrates using the sol–gel method. The formation of a pure Bi12SiO20 phase was observed at a temperature of 700°C. The Bi12SiO20 thin films, heat treated at 700°C for 1 h, had a dense microstructure with an average roughness (Ra) of 50 nm. The dielectric properties of the film were characterized by using both low- and microwave-frequency measurement techniques. The low-frequency measurements were conducted with a parallel capacitor configuration. The dielectric constant and dielectric losses were 44 and 7.5 × 10−3, respectively. The thin-film dielectric properties at the microwave frequency were measured using the split-post, dielectric resonator method (15 GHz) and the planar capacitor configuration (1–5 GHz). The dielectric constant and the dielectric losses measured at 15 GHz were 40 and 17 × 10−3, respectively, while the dielectric constant and the dielectric losses measured with the planar capacitor configuration were 39 and 65 × 10−3, respectively.