Abstract: The separation effects of lithium isotopes on the extraction with Sudan I(1-phenylazo-2-naphthol)-neutral ligand synergetic extraction systems are reportedin this paper. Different separation effects are observed when different kinds ofneutral ligands are used. Among them, the separation coefficient (α) of SudanI-TOPO-xylene/LiCl-LiOH is found to be 1.009±0.001. The heavy isotope~7Li is enriched in the organic phase. The contributions of different structuralchelating agents, synergetic agents and diluents to the separation effect arediscussed. It is shown that the chelating agent played an important role in theseparation effect, and the synergetic agent also exhibited obvious effect, while theshift of diluent didn't affect the separation coefficient significantly. The rates ofextraction and isotope exchange equilibrium are high and these systems areshown to be diffusion-controlled ones.