Chaiya and Tha Chana districts of Surat Thani province are located in the areas with high equivalent uranium at ground surface, which have been identified as sources of radon. A survey measurement of indoor radon concentrations was carried out in 248 houses, using CR-39 detectors in closed cups. All of the detectors were exposed to radon for forty days. After the exposure, the alpha tracks were made visible by chemical etching and counted manually under an optical microscope. The indoor concentrations in Chaiya district were found to vary from the minimum to the maximum of 4 Bq m”3 to 88 Bq m”3, respectively. In Tha Chana district, the concentrations of indoor radon were varied from the minimum of 4 Bq m~3 to the maximum of 159 Bq m”\ The geometric mean of indoor radon concentrations in Chaya and Tha Chana districts were found to be 26±2 Bq m”3 and 30±2 Bq m”3, respectively. The overall geometric mean in the surveyed areas was 28±2 Bq m”3 .Only in two houses (1%), the concentrations (151 and 159 Bq m3) were found to be higher than the action level recommended by the us EPA (148 Bq m”). Most houses (94%) have natural ventilation by keeping doors and windows opened during the daytime. This ventilation likely causes the low level of indoor radon concentrations.