Abstract: Based on the equilibrium plate model of the distillation type a set of the theoretical equations have been drawn for the isotope steady band with very small enrichment factor and the band being of a limited length. That is, we used the parameters , the enrichment factor of a couple of isotopes (ε), the molar fraction of the species to be enriched in the feed (x₀) and the adsorption band length expressed by the number of HETP (P) to determine the shape of the isotope steady band, the positions of the tails of the steady band and concentrations of the species to be enriched at the corresponding tails, the total enrichment of the species to be enriched in the isotope steady band,the variation of the total enrichment in the unsteady band due to the increase of the displacement distance and so forth. And some special situations were discussed.These main theoretical expressions are given by the Eqs. (2)-(10) in the paper, where Pis the position of some point from x=x₀ in the steady band; p', the position of the front of the steady band from x=x₀; x, the molar fraction of the species to be enriched at p; x_e, the molar fraction of the species to be enriched at P-p'; x_d, the molar fraction of the species to be enriched at p', E_e^l, the total enrichment of the species to be enriched in the enriched part of the steady band;E_d^l, the total depletion of the species to be enriched in the dopleted part of the steady band; E_e, the total enrichment of the species to be enriched in the enriched part of the unsteady band after displacing the distance of t numbers of HETP; t, displacing distance and a, an experimentally determined constant.The expressions in this paper bear some analogy to Conrard's in form, but Pis twice that of Conrard's because of the difference between the chromatographic motion models used.