The purpose of this study was to evaluate the basic unit of irrigation water on golf courses in Jeju Island. The amounts of rainwater and groundwater used on 20 golf courses have been monitored for this study. The characteristics of rainwater and groundwater from the selected 20 golf courses were analyzed based on the existing data that had been collected for three consecutive years from 2006 to 2009. The range of monthly irrigation water (groundwater + rainwater) used was about $13,200sim55,600;m^3$/month, with average of $36,600;m^3$/month. In the respects of the amount of annual water used, groundwater was recorded as $163,500;m^3$/year, and rainwater was recorded as $275,400;m^3$/year. Thus, the total annual irrigation water used was approximately $439,000;m^3$/year. The correlation (R2) between golf course lot size and average amount of monthly irrigation water used was 0.65, and the monthly basic unit per golf course area ($1,000;m^2$) was calculated as $60;m^3$.