A series of K3Gd1-x-y(PO4)2:xCe3+, yTb3+ phosphors are synthesized by the solid-sate reaction method. X-ray diffraction and photoluminescence spectra are utilized to characterize the structures and luminescence properties of the as-synthesized phosphors. Co-doping of Ce3+ enhances the emission intensity of Tb3+ greatly through an efficient energy transfer process from Ce3+ to Tb3+. The energy transfer is confirmed by photoluminescence spectra and decay time curves analysis. The efficiency and mechanism of energy transfer are investigated carefully. Moreover, due to the non-concentration quenching property of K3Tb(PO4)2, the photoluminescence spectra of K3Tb1-x(PO4)2:xCe3+ are studied and the results show that when x = 0.11 the strongest Tb3+ green emission can be realized.