High-dielectric-constant CaCu3Ti4O12 (CCTO) thin films were prepared on Pt/Ti/SiO2/Si(100) substrates by pulsed-laser deposition (PLD). The 480 nm thick polycrystalline films have preferred orientation and show obvious crystallization on the surface. The temperature dependence of dielectric constant and loss of the Pt/CCTO/Pt capacitors is comparable with that obtained in the epitaxial CCTO films grown on oxides substrates. We found that the dielectric properties are very sensitive to the postannealing atmosphere and temperature. Postannealing in nitrogen atmosphere produces larger low-frequency dielectric relaxation as the annealing temperature increases, while annealing in oxygen atmosphere at high temperature suppresses the relaxation and decreases the dielectric constant of the thin films. Such results are attributed to the presence of insulating grain-boundary barrier layers. © 2004 American Institute of Physics.