The potential energy difference of trivalent Pr/Nd complex ions with various α hydroxyl carboxylic acids were computed by molecular mechanics plus(MM +) model. Its relationships with the steric hindrance of the coordinating lignads and with the separation factor of Pr/Nd were discussed. It is found that the MM + model can be used for selecting and designing ligand as effective eluant, with which Pr 3+ and Nd 3+ can be separated by ion exchange chromatography. The potential energy difference of trivalent Pr / Nd complex ions with various α hydroxyl carboxylic acids were computed by molecular mechanics plus (MM +) model. Its relationships with the steric hindrance of the coordinating lignads and with the separation factor of Pr / Nd were discussed It is found that the MM + model can be used for selecting and designing ligand as an effective eluant, with which Pr 3+ and Nd 3+ can be separated by ion exchange chromatography.