The purpose of this study was to predict occurrence of earthquakes in Korea by measuring the concentration of radon radioactivity in the air and in the underground water. Two monitoring systems of radon concentration detection in the air were installed in Seoul, East Coast area, whereas of radon concentration in the underground water in Kyungju area during December, 1999 to June, 2001. The distribution of radon concentration in the air in Seoul is as follows : winter(10.10±2.81Bq/m3), autumn(8.41±1.35Bq/m3), summer(5.83±0.05Bq/m3) and spring(5.34±0.44Bq/m3), whereas the distribution of radon in the air in the East Coast area showed some difference as follows : autumn(14.08±5.75Bq/m3), summer(12.04±0.53Bq/m3), winter(12.02±1.40Bq/m3) and spring(8.93±0.91Bq/m3). In the meanwhile, the distribution of radon in the water is as follows : spring(123.59±16.36count/10min),winter(93.95±79.69counter/10min),autumn(68.96±37.53counter/10min) and spring(34.45±9.69counter/10min). The daily range of the density of radon concentration in Seoul and East Coast area was between 5.51Bq/m3-9.44Bq/m3, 7.15Bq/m3-15.27Bq/m3, respectively. Correlation of the distributions of radon concentrations in the air and in underground water with earthquake showed considerable variations of radon concentration before the occurrence of the earthquake. The results suggested that radon radioactivity seemed to be helpful for the prediction of the occurrence of earthquake.