Eu3+-doped epitaxial Gd2O3 ( 111) films with well-ordered crystalline structures were grown on oxidized Si ( 111) using the physical vapor deposition method. The mole fraction ( x) of Eu3+ in Gd2-xO3:Eu-x(3+) ranged from 0.02 to 0.22. The photoluminescence characteristics, measured at an excitation wavelength of 254 nm, showed that even at the very low Eu3+ concentration, x = 0.18, the D-5(0) -> F-7(2) transition occurred at the maximum 612-nm emission. Based on the critical distance calculated using the decay curves at 612 nm, we proved that the D-5(0) -> F-7(2) transition of the Gd2O3:Eu3+ originated from an electric dipole-dipole transition. In addition, the critical distance (R-c) was greater than that reported previously due to the perfectly crystalline film. This significantly decreases the mole fraction which maximize the photoluminescence intensity because the non-radiative transition is much lower than that of the chemically synthesized Gd2O3:Eu3+. (C) 2011 Elsevier B.V. All rights reserved.