The CaCu3Ti4O12 (CCTO) thin films were deposited on Pt/TiO2/Ti/SiO2/Si (100) substrates by pulsed laser deposition. The temperature dependences of dielectric constant were measured in the as-produced thin film samples, showing the so-called colossal dielectric constant (CDC) phenomenon. I-V and C-V characterizations evidenced that the electrode interface formed a Schottky barrier, whose depletion capacitance resulted in the observed CDC in CCTO thin films, while the interior of the material was found to be a p-type doped semiconductor. The CDC state of CCTO can be switched off by annealing the samples in air at 600 to 700 degrees C. In the non-CDC state, the intrinsic dielectric constant of CCTO increases on cooling, showing an incipient ferroelectric feature. XPS depth profiles indicate that the p-type doped state in the bulk corresponds to partially reduced Cu++ ions combined with oxygen vacancies.