For decreasing the HETP value of a column packed with a crown ether adsorbent for the separation of lithium isotopes by displacement chromatography, adsorbents were prepared using porous silica beads having diameters of 60, 100, and 250 μm and benzo-15-crown-5 ether. The HETP values of the packed column were estimated by chromatographic experiments by data obtained from breakthrough curves. The HETP values decreased with the particle diameter of the adsorbent, and an HETP value of 0.16 mm was obtained for a particle diameter of 60 μm. Numerical simulation results suggested that the HETP value can be possibly reduced to the order of 10 μm by using an adsorbent with a particle diameter of 5 μm.