Abstract: Separation factors of lithium isotopes at 20℃ were determined in the extraction systems of B15C5-CHCl_3/LiBr-H_2O, The initial concentration of LiBr was controlled in the extent of more than 2mol/1. It may be established that the increase of LiBr concentration causes a remarkable increase of the separation factor. The essence of this effect due to the change in salt concentration was discussed in connection with examination of relevant phenomena in literature,, It can be concluded that the relationship between a and Cm, the concentration of lithium salt, is dependent on K_Q and K_P, where K_Q and K_P express lithium isotope exchange equilibrium constants between Li-crownether complex and hydrated lithium ion for lithium concentration less than 1—2 mol/1, and that between lithium salt ion pair and hydrated lithium ion for lithium concentration more than 2mol/1 in aqueous phase respectively.