The change of the defects in PbWO4 crystals before and after annealing was studied by positron annihilation lifetime spectrum. It was found that the lifetime τ
2
turned short and the positron capture ability K increased after oxygen annealing.The results for the case of vacuum annealing were the reverse for that of oxygen annealing. Meanwhile
the main emission peak of the crystal after oxygen annealing shifted from 440nm to 485nm
but the peak for the case of vacuum annealing remained at 440nm
i.e. similar to that of an untreated crystal. From the results affected by different treatments
it is proposed that WO
3
+O- produced by lead vacancy leads to the green luminescence in PbWO4 crystal.