Biosorption has been developed as an effective and economic method to treat wastewater containing low concentrations of metal pollutants. In this study, a bacterium, Citrobacter freudii, was used as a biosorbent to adsorb uraniumions. The factors which influence the adsorption of uranium by Citrobacter freudii were investigated, including pH, the strain dosage and the initial concentrate of uranium. The results showed that the adsorption efficiency increased with the increased of pH in the beginning of the adsorption and reached its maximum at pH 6; when the strain dosage and the initial concentrate of uranium were 6.0g/L and 20mg/L, respectively, the adsorption efficiency reached 93.89% and 94.68%, respectively. The authors investigated the active sites of bacteria for biosorption and the results proved that carboxyl in the cell wall played an important role in biosorption.