The isotopic enrichment process of iron isotopes by a chemical exchange reaction of liquid-liquid extraction using a crown ether, dicyciohexano-18-crown-6 was investigated. An enrichment factor of54Fe relative to56Fe WHS ε54.56= 0.000 ± 0.003, while that of58Fe to58Fe was ε 58,56 = -0.032 ± 0.004, instead of then-similar mass differences. This showed the breakdown of the Bigeleisen-Mayer approximation which states the isotope effect is proportional to the mass difference in the chemical exchange reaction. The difference of nuclear size and shape, namely the field shift in atomic spectrum was found to dominate the enrichment factors of the iron isotopes. Contribution of the nuclear size and shape was -0.199 to the enrichment factor observed for57Fe to56Fe, ε 5756 = -0.0055 ± 0.0004, and that of the nuclear spin was 0.049.