Green-emitting phosphors of Ca3 Y2(Si3 O 9)2: Ce3+, Tb3+ have been synthesized by solid-state reaction. The luminescence properties were characterized by photoluminescence (PL) and photoluminescence excitation spectra, reflection spectra, and a detection system using an integrating sphere equipped with a spectrofluorometer. With Ce3+ co-doping as a sensitizer, the PL intensity of Ca3 Y2(Si3 O9)2: Ce3+, Tb3+was found to increase dramatically by a factor of 5.8. For comparison, the emission intensity of composition-optimized Ca3 Y2(Si3 O 9)2: Ce3+, Tb3+is 126% of green commodity, ZnS:Cu,Al. The quantum efficiency of Ca3 Y 2(Si3 O9)2: Ce3+, Tb 3+is as high as 77% compared to ZnS:Cu,Al (53%). The energy transfer from Ce3+to Tb3+is also of resonant type via a dipole-dipole mechanism with the energy-transfer critical distance of 6.78 Å. By utilizing the principle of energy transfer and appropriate tuning of activator contents, we are currently evaluating the potential application of Ca3 Y2(Si3 O9)2: Ce 3+, Tb3+as a highly efficient UV-convertible phosphor.