Long-term measurement due to radiotoxicity of radon and its progeny had been done to determine indoor Rn-222 concentration level in 30 houses in Mahallat. Passive detection method by using CR-39 Solid State Track Detectors (Raduet detectors as a subtype of the standard RSFS detectors) was dedicated to combination detection of indoor radon and thoron activity simultaneously in living and bed rooms for a period of 120 days. Study shows a various range of radon activity in connection with house age from 22.6±2.3 Bq.m-3 for apartment in 2nd floor to 350.3±25.8 Bq.m-3 for walk-up basement and with an average of 158.25 Bq.m-3 for all dwellings. Exposure to radon and its daughters was calculated for lowest, highest and mean radon concentration by 0.09 WLM.y-1 (0.79 mSv.y-1), 1.36 WLM.y-1 (12.3 mSv.y-1) and 0.62 WLM.y-1 (5.5 mSv.y-1). In light of results, according to EPA recommendation and ICRP statement, the excess annul lung cancer risk from radon and its progeny for three categories of Current Smokers (CS), Ever Smokers (ES), Never Smokers (NS) were estimated by 0.09%, 0.06%, 0.01% and 0.03% for general population of all age, respectively.