The zinc isotope effect in a liquid-liquid extraction system using dicyclohexano-18-crown-6 was investigated. The enrichment factor for a unit difference of mass number was epsilon(u) = 0.018 as a maximum, which is greater than that for magnesium isotopes : The enrichment factor to eliminate Zn-64 from Zn-66, Zn-67, Zn-68, or Zn-70 is over 0.036. The isotope with an odd mass number, Zn-67, behaved differently from those with even mass numbers. This odd/even isotope effect was epsilon(O/E) = 0.056. From the Values of epsilon(u) and epsilon(O/E), it was found that the crown ether separated the zinc isotopes more effectively on the basis of an odd or an even mass number than of mass difference. The separation factors vary with the concentrations of salt and/or conjugated acid in the initial aqueous phases of extraction. The optimal concentration necessary to obtain the largest separation factor had components of 2.0 M ZnCl2 and 1.0 M HCl. The large value of epsilon(u) for the high atomic number zinc and the notable epsilon(O/E) make it clear that the vibration frequencies of the intramolecular bonds should have an isotope shift which is recognized in the orbital energy of the atoms.