Cheju island formed by Quarternary volcanism has highly permeable hydrogeological environment. To meet the increasing demand of water in the island, many groundwater wells have been developed. The environmental isotopes (oxygen-18, deuterium, tritium) study and chemical analysis on water samples from the eastern part of the island were carried out to ascertain whether groundwater in the eastern part of the island was contaminated by sea water. The result of tritium analysis indicated fast infiltration of meteoric water into underground and rapid mixing process between rain water and groundwater. The results of oxygen-18 and deuterium analysis demonstrated that most of the wells in the eastern part of the island were influenced by sea water intrusion. Based on chemical analysis of water samples most of groundwater in study area were classified into Na-Cl type and showed high chloride/bicarbonate ratios. Sea water intrusion in the northeastern part of the island has proceeded to inward island at least 3 km from the coastal line, and in the southeastern part about 700 m. (Author).