The objective of this study was to identify the primary source of radon in Seoul subway stations, and to investigate a relationship between geology and radon. Especially, we expected that the granite areas would have substantially high levels of radon in subway stations. The indoor radon concentrations in subway stations were lognormally distributed. The geometric mean and geometric standard deviation of indoor radon concentration were 48.11 Bq/㎥ and 2.15, respectively. Indoor radon concentrations of eight measuring sites exceeded U.S. EPA criteria (148 Bq/㎥). The geological structure of the subway station regions under this study is characterized by biotite granite, alluvium, banded biotite gneiss and diluvium. Results indicate that bedrock geology can account for a significant portion of the indoor radon in subway stations. Indoor radon concentrations of one subway station were higher than those of other stations. The bed rock in this particular subway station was that of alluvium. We assumed that the unusual increase in measured radon concentration should be related mainly to the existence of the near inferred fault zone (p<0.0001). We selected ten subway stations with homogeneous bedrock type in order to compare radon concentrations of each basement level. There was a significant difference in radon concentration, depending on the basement levels in subway stations (p<0.05).