Abstract: In this work, the enrichment behavior and mechanisms of U(Ⅵ) by Enterobacter sp. X57, a novel bacterial strain isolated from uranium contaminated sites, under different environmental conditions were investigated to develop superior microorganisms for uranium-contaminated environments bioremediation. The results show that the enrichment of U(Ⅵ) by Enterobacter sp. X57 depended on the cellular metabolic activity, and was also influenced by the contact time, pH, temperature, initial U(Ⅵ) concentration and coexisting ions. Under the conditions of initial concentration of U(Ⅵ) of 100 mg/L, temperature of 303 K and pH=7.00, the maximum enrichment capacity of Enterobacter sp. X57 is about 175.52 mg/g (dry weight). Kinetics results indicate that the enrichment process of U(Ⅵ) by Enterobacter sp. X57 involves chemical adsorption and intra particle diffusion. The analyses of FT-IR, SEM, TEM, EDS and XRD further reveal that uranyl in solution could rapidly bond with the carboxyl, amino, and phosphoryl groups of Enterobacter sp. X57, and diffuse into cell periplasmic space, depositing as U(Ⅵ)-phosphate minerals on the cell plasma membrane. The results indicate that the enrichment of uranium by enterbacter sp. X57 is realized by the synergism of biosorption and biomineralization.