Green-emitting phosphors of Ca3Y2(Si3O9)(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 Ca3Y2(Si3O9)(2):Ce3+,Tb3+ was found to increase dramatically by a factor of 5.8. For comparison, the emission intensity of composition-optimized Ca3Y2(Si3O9)(2):Ce3+,Tb3+ is 126% of green commodity, ZnS:Cu,Al. The quantum efficiency of Ca3Y2(Si3O9)(2):Ce3+,Tb3+ 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 A degrees. By utilizing the principle of energy transfer and appropriate tuning of activator contents, we are currently evaluating the potential application of Ca3Y2(Si3O9)(2):Ce3+,Tb3+ as a highly efficient UV-convertible phosphor.