A direct relation between dielectric constant and surface potential at grain boundary in giant permittivity CaCu₃Ti₄O₁₂ (CCTO) thin films doped with various dopants of Zn²⁺, Ga³⁺, Mn²⁺ and Ag¹⁺ is proposed here. The CCTO-based materials have been one of emerging dielectric candidates due to a giant dielectric constant of >10⁴, which is different with typical high k materials such as BaTiO₃ or Pb(Zr,Ti)O₃ in that the CCTO is based on the interfacial (or space charge) polarization mechanism (instead of permanent dipolar polarization). Accordingly, the electrical status of grain boundary has been recognized as the main cause for the exceptionally high permittivity. As a highlight, we suggest a nearly linear relation between the dielectric constant and grain boundary potential, with experimental evidences. As an experimental result for four different choices of dopants, Zn²⁺, Ga³⁺, Mn²⁺ and Ag¹⁺ in terms of dielectric properties, chemical states of involved defects such as Cu¹⁺, Ti³⁺ and oxygen vacancy are proven to dictate the dopant-dependent dielectric performance. Particularly, the case of Ag-doping exhibited the best dielectric constant as related to the highest grain boundary potential. A direct observation of the grain boundary region by using spherical aberration-corrected scanning transmission electron microscopy (Cs-STEM) with the simultaneous electron energy-loss spectroscopy (EELS) reveals the changed chemical states across the grain boundary.