This study investigated the indoor radon concentration of 44 elementary schools in Gyeongsang-do from June 2008 to May 2009. The results obtained from this investigation are as follows. As for distribution of concentration based on seasons, the radon concentration was 77.4Bq/m3 in winter, 71.8Bq/m3 in autumn, 47.8Bq/m3 in spring and 40.4Bq/m3 in summer of Gyeongsangnam-do. And Gyeongsangbuk-do was 155.4Bq/m3 in winter, 124.3Bq/m3 in autumn, 82.7Bq/m3 in spring and 58.0Bq/m3 in summer, showing the highest concentration in winter. As for difference in radon concentration according to whether there is basement, concentration of schools having basement was 37.2Bq/m3, that of schools having no basement was 62.1Bq/m3 in Gyeongsangnam-do. In Gyeongsangbuk-do, schools having basement showed 53.9Bq/m3 of concentration and schools having no basement 124.7Bq/m3. Schools having no basement tend to show higher concentration. Indoor radon concentration according to the constructing year was 64.5Bq/m3 in schools built before 1990, 34.9Bq/m3 during 1990s and 32.8Bq/m3 during 2000s in Gyeongsangnam-do, and 110.5Bq/m3, 83.5Bq/m3 and 48.3Bq/m3 in Gyeongsangbuk-do respectively. Keywords : radon, school, seasonal, indoor air, basement