Background: Inhalation of radon and its short-lived decay products is one of the ‎most significant sources of exposure to natural radiation. Radon is the second cause ‎of lung cancer in the populations. The present study was carried out under the projects of ‎national radon, with the aim of determining the concentration of indoor in the city of Qom ‎located in the central semi-arid region of Iran.‎ Materials and Methods: Radon measurements were carried out in 123 dwellings ‎using passive sampling with CR-39 detectors for 90 days. The map of radon ‎concentration distribution was prepared using Arc GIS software and the statistical ‎analysis was performed with SPSS version 20. Results: Indoor radon concentrations in Qom dwellings ranged from 15–259 Bq ‎m^-‎‎3‎. The arithmetic mean of indoor radon concentrations on basement, ground floors, ‎first floors and second and upper floors were ‎123.43, 87.94, 63.72 and 40.69‎‏ ‏Bq m^-3, ‎respectively. Conclusion: A correlation was found between the distances from fault zones and measured ‎indoor radon concentration. In most of cases, radon values were lower in well- ‎ventilated dwellings in comparison with poorly-ventilated ones. Moreover, high radon ‎concentration levels were observed in basements. The results indicated that in 30 ‎places (24.3% of cases), the radon concentrations were higher than the reference levels ‎recommended by the World Health Organization (100 Bq m^-3).