Study of indoor radon/thoron has been carried out in some dwellings of Firozabad, Uttar Pradesh, India using LR-115 type II plastic track detectors. Radon is an invisible radioactive gas that occurs naturally in the environs. It comes from the natural breakdown of uranium in soils and rocks. Lung cancer risk depends upon the concentration of radon and their decay products in air above recommendation level. In the present study the value of concentration of radon ranges from 31.5 to 141.4 Bqm-3 whereas the value of thoron concentration ranges from 9.3 to 75.2 Bqm-3. The value of radon progeny ranges from 3.4 to 15.3 Bqm-3 while the value of thoron progeny ranges from 3.3 to 27.3 Bqm-3. The inhalation dose varies from 0.15 to 0.71 mSv.y-1.The annual exposure (Rn + Tn) in terms of WLM ranges from 0.24 to 1.46 while annual exposure (Rn + Tn) in terms of mjhm-3 ranges from 0.85 to 5.17 with an average of 2.9. The life time fatality risk of the residents of the study area ranges from 0.72 × 10-4 to 4.38 × 10-4 with an average value of 2.5 × 10-4. The annual effective dose ranges from 0.93 - 5.66 mSvy-1 with an average value of 3.2.