BaTi 4 O 9 thin films have been prepared by RF magnetron sputtering on the Pt/Ti/SiO 2 /Si substrates and the dielectric properties of the BaTi 4 O 9 film have been investigated at microwave frequency range. The homogeneous BaTi 4 O 9 thin film was obtained when the film was grown at 550°C and rapid thermal annealed (RTA) at 900°C for 3min. The circular-patch capacitor (CPC) was used to measure the microwave dielectric properties of the film. The dielectric constant (ɛ r ) and the dielectric loss (tanδ) were successfully measured up to 6GHz. The ɛ r of the BaTi 4 O 9 thin film slightly increased with the increase of the film thickness. However, the tanδ decreased with increasing the thickness of the film. The ɛ r of BaTi 4 O 9 thin film was similar to that of the BaTi 4 O 9 ceramics, which is about 36–39. The tanδ of the film with 460nm thickness was very low approximately, 0.0001 at 1–3GHz. Since BaTi 4 O 9 film has a high ɛ r and a low tanδ, the BaTi 4 O 9 film can be used as the microwave devices.