We report angle-resolved photoemission spectroscopy (ARPES) results of A-site ordered perovskite CaCu3Ti4O12. We have observed the clear band dispersions, which are shifted to the higher energy by 1.7 eV and show the band narrowing around 2 eV in comparison with the local density approximation calculations. In addition, the high energy multiplet structures of Cu 3d 8 final-states have been found around 8 -13 eV. These results reveal that CaCu3Ti4O12 is a Mott-type insulator caused by the strong correlation effects of the Cu 3d electrons well hybridized with O 2p states. Unexpectedly, there exist a very small spectral weight at the Fermi level in the insulator phase, indicating the existence of isolated metallic states.A-site ordered perovskite CaCu 3 Ti 4 O 12 (CCTO) has generated considerable interest due to the extremely high dielectric constant ( ) as high as 10 4 -10 5 over a wide range of temperature from 100 to 600 K, which holds a promise for high performance capacitor [1]. Prior to applications, there have been many studies to identify the intrinsic mechanism of the high . Although the consistent conclusion of the origin has not been established yet, it has been widely accepted that the high would come from defects and/or disorder structures, e.g. a relaxor like dipole fluctuation in nanosize domain [2], an internal barrier layer capacitance [3], and the nanoscale disorder of Ca and Cu-site [4]. Generally, the origin of the high of CCTO has been considered to be different from that of conventional ferroelectric materials, because of the absence of structural transition accompanying with the abrupt change of around 100 K [5,6]. Therefore, in order to understand these intrigue physical properties, the electronic structures should be clarified experimentally. In particular, relations between the electronic structure and the strong correlation effects are central issues [7][8][9]. For example, the high and an insulator phase of CCTO can not be explained by theoretical calculations within the local density approximation (LDA), which are not considered to properly treat strong correlation between electrons. The strong correlation effects can be also expected from the crystal structure of CCTO, which contains the CuO 4 plane units in similar to the CuO 2 plane of the high-T c cuprates as shown in Fig. 1 (b) [10]. Recently, it has been reported that a family compound, CaCu 3 Ru 4 O 12 , shows the heavy fermion behavior and the non-Fermi liquid, supporting the importance of the strongly correlation effects [11]. Hence, it has been believed that CCTO would be a Mott-type insulator, even though the experimental band dispersions have never been observed.Here, we first report the clear observation of band dispersions of CCTO by the angle-resolved photoemission spectroscopy (ARPES) measurements. ARPES experiments were performed at the beamline BL5U of UVSOR, using photon energies (hν) from 30 to 93 eV. Measurements were carried out at room temperature (T = 300 K) in a vacuum better than 2 × 10 −8 Pa. Total energy res...