Abstract: The formation of Am colloid in deionized water, Beishan groundwater, and ultrafiltrated groundwater was studied and the influence of pH, ionic strength, and americium initial concentration on the colloidal behavior of Am was investigated under anoxic condition in this paper. Experimental results indicate that formation of Am colloid is influenced strongly by pH and ionic strength. The amount of Am colloid increases with the increase of pH value (pH= 6-9 ). The original concentration of Am has no significant effect on the formation of americium colloid if it is far lower than the solubility of Am, while the natural colloid and anions existing in groundwater can strongly affect the colloidal behavior of Am. The saturation indices of the solid phase which controlled Am solubility under different experiment conditions were calculated and used in the discussion of the experiment results.