Magnesium isotopes effects were investigated by chemical ion exchange using synthesized 2-aminomethyl-18-crown-6 (AM18C6) bonded Merrifield peptide resin. It was found that separation factors larger those reported previously were obtained, and the hydration and isotope mass effects are more significant than that of the complexation. The capacity of the crown ion exchanger was 2.3 meq/g dry resin. The adsorption capacity of the resin for magnesium ion was 26.8 mg/g dry resin at pH 7. The heavier isotopes of magnesium were enriched in the solution phase, while the lighter isotopes were enriched in the resin phase. The separation factors of Mg-24- Mg-25, Mg-24- Mg-26, and Mg-25-Mg-26 were 1.0085, 1.0162, and 1.0081, respectively.