Abstract: Expounding the equality between the equation based on the rate theory and that on the theory of equilibrium and its condition, using the micro-resins with the diameters of 30-40μ and 40-60μ respectively and using DTPAas displacer to separate rare earths by high pressure ion exchange chelate displacement chromatography at 75±3℃, the report has experimentally shown that the film diffusion of ions is the control step and decides HETP, which provides the theoretical base of employing Displacer I (a displacer with certain chelate agent concentration, pHand various concentrations of neutral salt) into high pressure ion exchange ehelate displacement chromatography.By examing and discussing the dependence of HETPupon the experiment conditions we have proved Snyder's formula is suitable for high pressure ion exchange chromatographic process.