The underlying physical mechanism of the so-called colossal dielectric constant phenomenon in CaCu3Ti4O12 CCTO thin films were investigated by using semiconductor theories and methods. The semiconductivity of CCTO thin films originated from the acceptor defect at a level 90 meV higher than valence band. Two contact types, metal-semiconductor and metal-insulatorsemiconductor junctions, were observed and their barrier heights, and impurity concentrations were theoretically calculated. Accordingly, the Schottky barrier height of metal-semiconductor contact is about 0.8 eV, and the diffusion barrier height of metal-insulator-semiconductor contact is about 0.4–0.7 eV. The defect concentrations of both samples are quite similar, of the magnitude of 1019 cm−3, indicating an inherent feature of high defect concentration.