Er3+,Tb3+-codoped gadolinium oxysulfide phosphor was prepared using solid state reaction in vacuum. The structure and morphology of the resulting particles were characterized by X-ray diffraction and transmission electron microscope whereas their optical properties were monitored by photoluminescence spectroscopy. The power dependence on color-tunable and upconversion luminescence were also obtained in this paper. Color emission of the synthesized phosphors can be tuned from green to yellow due to Tb3+ ions doping by varying the excitation power from 1.0 W to 2.0 W. The first principles calculation method was employed to study the crystal, electronic structure and optical properties of Gd2O2S:Er crystals, so it was further confirmed that doping of Tb3+ ions played significant role of energy transfer rather than crystal structure of the host lattice. (C) 2015 Published by Elsevier B.V.