Biological removal of lead was observed in this study during treatment of synthetic solutions of this heavy metal by a sample of waste activated sludge (WAS) having MLSS concentration of 10000 mg/L. The objective was determining the capability of WAS in removal of lead in three different conditions of treatment: without any aeration and feeding, by simultaneous aeration and feeding and by aeration without feeding. Besides, the effect of initial metal concentration and contact time in these treatment stages were determined.Results showed that the efficiency of lead removal was reduced by increasing the initial metal concentration, but the changes of contact time had resulted in different responses depending to the test condition. Moreover, it was found that the effects of aeration and feeding in increasing the efficiency of treatment were more pronounced for the stage of treating lower concentrations of lead. The maximum removal of lead in the first step (without aeration and feeding) was 55.2 % in contact time of 75 minutes, in the second step (without feeding and by 12 hours aeration) 63.3% and third step it was 94.2 % in contact time of 48 hours .The initial lead concentration in these experiments was 5 mg/L.