Stability constants of all rare earth ions (REE) with fulvic acid were determined using solvent extraction coupled with ICP-MS detection. REE pattern of the stability constant of REE fulvates shows a similar pattern to REE-carboxylate complex, suggesting that carboxylic group is the main binding site of REE in humic substances. The stability constants for all REE are indispensable for the understanding of REE patterns in natural aquifer.