PWO crystals doped with yttrium were grown with the Bridgman method in platinum crucible and by using an indigenously developed resistive heating fuace. After an exposure of γ-ray from a 60 Co source, with the dose rate of 15 rad/h for 20 h, the light output increases for about 15%, accompanied with vanishing of an optical absorption band at 420 nm. The excitation and emission spectra of PWO crystals were measured before and after irradiation with different dose rates. The optical absorption band at 420nm was also found in the PWO sample annealed in oxygen-rich atmosphere. It is suggested that the absorption band at 420nm is related to Pb3+ point defects existing in the PWO crystal. The unusual change of light output after irradiation probably results from the transformation of lead ions from Pbs+ to Pb2+.