Polycrystalline CaCu3Ti4O12 thin films were deposited on Pt(111)/Ti/SiO2/Si substrates using radio frequency magnetron sputtering. The phase formation and the physical quality of the films were crucially dependent on the substrate temperature and oxygen partial pressure. Good quality films were obtained at a substrate temperature of 650 degrees C and 4.86 Pa total pressure with 1% O-2. The dielectric constant (similar to 5000 at 1 kHz and 400 K) of these films was comparable to those obtained by the other techniques, eventhough, it was much lower than that of the parent polycrystallme ceramics. For a given temperature of measurements, dielectric relaxation frequency in thin film was found to be much lower than that observed in the bulk. Also, activation energy associated with the dielectric relaxation for the thin film (0.5 eV) was found to be much higher than that observed in the bulk ceramic (0.1 eV). Maxwell-Wagner relaxation model was used to explain the dielectric phenomena observed in CaCu3Ti4O12 thin films and bulk ceramics. (C) 2007 Elsevier B.V All rights reserved.