Visible up-conversion luminescence at 528, 550 and 660 nm have been observed in Er3+-doped PbWO4 crystals under infrared excitations at both 970 and 800 nm. The crystal shows more efficient up-conversion emissions under pumping by 800 nm laser diode (LD) than that obtained under LD 970 nm. Higher is the doping concentration of Er3+ in PbWO4 crystal, stronger is intensity of up-conversion luminescence. Different from the up-conversion luminescence, normal luminescence of Er3+ under excitation of X-ray and Xenon lamp at 379 nm were quenched in PbWO4 crystals at a higher Er3+ doping concentration. Pumping power measurements were also carried out. Combined with the energy level structure of Er3+ and the decay kinetics of the green emissions, the possible up-conversion luminescence mechanism has been analyzed and explained, which were closely related to the Er3+ doping concentrations. In PbWO4 crystals with low Er3+ doping, the up-conversion is populated mainly through excited-state absorption; and through energy-transfer processes at higher Er3+ ions doping levels. (c) 2004 Elsevier B.V. All rights reserved.