Abstract: The lithium isotope separation ability in extractive process of LiSCN with CHCl3 solution of p-methylphenoxy linked biscrown ether is determined. The characteristic separative ability εp is 1. 74×10-2 for this system at 25℃, and smaller than that of B15C5 under the same conditions. Thus it can be seen that the lithium isotope separation ability of monocyclic crown ether will decrease as polybasic product is formed from the monomers.