Objective: This study aims to characterize airborne radon and thoron levels (Bq/㎥) generated from working environments in three subway stations in Seoul. Method: A radon and thoron detector (EQF3220) was used to monitor real-time airborne radon and thoron levels (Bq/㎥) and their daughters (Bq/㎥) every two hours. They were monitored not only in the driver’s cabin of seven circulation lines, but also three offices, platforms, and water pump reservoirs in the three stations. Results: The average levels of radon and thoron were 67.9 Bq/㎥ (range; 7.2-619.4 Bq/㎥) and 44.4 Bq/㎥ (range; 4.3-819.2 Bq/㎥), respectively. Notably, higher than legal airborne radon levels (600 Bq/㎥) were frequently monitored in the driver’s cabin of seven circulation lines. Airborne radon levels monitored in the platforms and administrative offices were found to be over 100 Bq/㎥. The average equilibrium factors (F) were 0.12 and 0.06, respectively. The percentages detected were found to be 84.9 for radon and 72.4 for thoron, respectively. Conclusions: Significant airborne radon and thoron levels were frequently found to be generated in subway facilities including water reservoirs, platforms and driver’s cabins. Further study is necessary to thoroughly investigate airborne radon and thoron in all subway stations and to devise proper measures.