The photoluminescence properties of BaGdB9O16:Tb3+ under vacuum UV and UV excitation were investigated, and visible quantum cutting through downconversion was observed in this phosphor. Upon Tb3+ 4f(8) -> 4f(7)5d(1) excitation, one UV photon absorbed by Tb3+ could be split into more than one visible photon emitted by Tb3+ through cross relaxation and direct energy transfer. To rationalize the quantum cutting effect, the proper mechanism was proposed. According to calculations, the maximal quantum efficiency was up to 161.3%. The quantum efficiency variation of BaGd1-xB9O16:Tb-x(3+) would depend on the concentration of Tb3+. Quantum efficiency would increase with the increased concentration of Tb3+ and could not be identified until the D-5(3) -> F-7(J) transitions of Tb3+ quenched completely. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3454192] All rights reserved.