About 87% of the radiation dose received by mankind is due to natural radiation sources and the remaining 13% by anthropogenic radiation [1]. The worldwide average natural radiation dose to humans is about 0.3-1.0mSv/year which is four times higher than the world average of artificial radiation exposure [2]. The most important component of natural radiation exposure to the public is due to the inhalation of indoor radon, thoron and their decay products which constitute about 54% of natural radiation [3]. As radon/thoron gases are inhaled, densely ionizing alpha particles emitted by the short-lived decay products of radon/thoron can interact with biological tissue in the lungs leading to DNA damage and results into lung cancer [4]. Since majority of the residents use to spend most of the time at indoors, the knowledge of the indoor concentration of radon, thoron and their progenies is of utmost importance.To calculate the annual indoor inhalation dose of radon and thoron, their indoor concentration, progenies concentration and their equilibrium factors must be calculated first.In the present study, passive measurement technique has been followed for measurement of indoor radon and thoron concentration, using Solid State Nuclear Track Detector (LR-115 Type-II) in Pin-Hole Based Twin Cup Dosimeters with single entry developed by Bhabha Atomic Research Centre (BARC), Mumbai [5]. To measure the progenies concentration, deposition base Direct Radon/Thoron Progeny Sensor (DRPS/DTPS) were used [6][7][8]. Dwellings were selected according to the gamma level measured using Gamma Survey Meter and the building type. The GPS co-ordinates have also been recorded for each dwelling in which dosimeters and DRPS/DTPS were installed for proper location assessments. Study AreaThe geographical location of the study area is Saiha and Lawngtlai districts of Mizoram, India within Latitude-22 0 29 ' 28.3 " to 22 0 36 ' 59.4 " and Longitude 92 0 59 ' 14.3 " to 92 0 52 ' 22.0 " with altitude ranging from 1866±10 feet to 3807±10 feet from the sea level. Methods Indoor radon & thoronThe LR-115 type-II plastic track detectors (SSNTDs) kept in the Dosimeter was deployed