Abstract: Some parameters have been determined for lithium isotope separation in the liquid-liquid extraction system H2O/C6H5I, using 4-tert-butyl-benzo-15-crown-5 as complexing agent. It is found that the relation between(α-1 )and (1-p) is linear in the extraction systems of lithium, using crown ethers and cryptands as complexing agent, where α denotes isotopic separation factor and p the concentration ratio,Li/polyether , in the organic phase. In view of △Go (free energy change for the exchange reaction) and εp (enrichment factor obtained when p is equal to 100%). it can be seen that the isotope separation ability of relative systems decreases in the order: cryptand 2, 2,1-CHCl3 /CF3 COOLi-H2O(or 2,2, 1-resin/LiI-methanol), 4-tert-butylbenzo-15-crown -5-C6H6I/LiSCN-H2O, benzo-15-crown-5-CHCl3/LiI-H2O.