The thoron survey was performed in granite quarries around Bangalore rural district in the scope of a lung cancer epidemiological study. Long duration measurements of outdoor thoron and its progenies concentrations were made around granite quarries of Bangalore rural district by using solid state nuclear track detector based double chamber dosimeters (LR-115, Type-II Plastic track detector) during summer and winter period (2006-07). The thoron concentration in granite quarries varies from 30 to 160 Bq.m-3 with amedian of 84.5 Bq.m-3 and its progenies varies from0.1 to 4.0mWLwith amedian of 1.2mWLrespectively. Higher concentrations of thoron and its progenies were observed where the granite rocks are exposure to the surface and are containing higher concentration of thorium. The concentration of thoron and its progenies and equivalent effective dose are tabulated with light limit of ICRP limits.