Radon concentration in dwellings in Mamit town area has been observed for about two years from May 2009 to February 2011. Seasonal variations of these radon concentration levels were observed for a year and after which landslide took place in a part of Mamit town and indoor radon concentration levels were also measured in landslide affected area. Pre- and post-landslide radon data are compiled and compared. For this, indoor radon concentration measurement, a time integrated method was employed by using a solid state nuclear track detector (SSNTD) type (LR-115films) kept in a twin cup dosimeter. It was found that the radon concentration level during pre-landslide was more than that of the post-landslide level. This behaviour is similar to the changes of concentrations in soil-gas radon which is associated with the earthquake activities as observed in Garhwal Himalayas. The radon content in the soil gas measured using RAD7 even after the landslide phenomena was found to be not so high.