The non-doped and 0.5, 1.0, 2.0, and 5.0 similar to mol% Er3+-doped 0.75Pb(Mg1/3Nb2/3)O3-0.25PbTiO3 (called PMN-PT) transparent ceramics have been fabricated. The optical absorption spectra, photoluminescence excitation and emission spectra, and the up-conversion luminescence spectra under the excitation of 980 similar to nm laser were investigated. The up-conversion luminescence of Er3+-doped PMN-PT transparent ceramics changes from yellowish green to reddish orange with increasing the Er3+ doping concentration. The dependence of visible up-conversion luminescence on the laser power was measured. The up-conversion luminescence saturation is observed for Er3+ 5.0 similar to mol%-doped PMN-PT. A possible up-conversion luminescence mechanism is proposed, which depends on Er3+ concentration. The up-conversion emission is mainly due to excited state absorption for lightly Er3+-doped PMN-PT transparent ceramics, while the energy-transfer processes are for highly doped ones.