The chemical forms of uranium in an artificial seawater are determined by use of the ion-association model. The data on the stoichiometric association constants of the ion pairs are used in calculation to evaluate the distribution of the major ionic species. Also, the more accurate values of the thermodynamic stability constants are used for the uranyl complexes. The calculation indicates that a dominant species of uranium is UO2(CO3)4-3in artificial seawater at 25°C.