Based on Kronig-Penney model, the changing tendency of band gap or a particular level with the volume deformation in crystalline materials was derived. Based on the tendency, the zero-phonon charge-transfer (CT) energy Ezp was deduced to be decreased when the size of Y2 O3 :Eu3 + phosphor decreases in nano-scale. In addition, the rigidity decrease of lattice environment in Y2 O3 :Eu3 + nano-phosphor leads to the enlargement of CT state coordinate offset, an optical center would reach a higher vibration level Evib in CT excitation. The increasing magnitude in Evib is smaller than the decreasing magnitude of Ezp when the size of Y2 O3 :Eu3 + phosphor decreases in nano-scale. As a result, the CT energy ECT is decreased, and the CT excitation spectrum shifts to lower energy.