This study was performed to investigate the concentration of $PM_{10}$ and $PM_{2.5}$ in inside train and platform of subway 1, 2, 4 and 5 in Seoul, KOREA. $PM_{10}$, $PM_{2.5}$, temperature, humidity and carbon dioxide were monitored using Portable Aerosol Spectrometer at afternoon (between 13:00 and 16:00). The concentrations of $PM_{10}$ and $PM_{2.5}$ in inside train were monitored to be higher than those measured in platform. In addition, $PM_{10}$ concentration in both platform and inside train were found to be greatly higher than range of from 35 ${mu}g/m^3$ to 81${mu}g/m^3$ in ambient air reported by Ministry of Environment. This study found that there were many inside train in subway 1, 2, 4 line where exceeded 150 ${mu}g/m^3$ of Korean PM10 standard. The average percentage that exceeded PM10 standard was 83.3% in line 1, 37.9% in line 2 and 63.1% in line 4, respectively. In particular, most of inside train in subway line 1 were over PM10 limit. PM2.5 concentration ranged from 77.7 ${mu}g/m^3$ to 158.2 ${mu}g/m^3$, which were found to be greatly higher than ambient air PM2.5 standard promulgated by United States Environmental Protection Agency (US-EPA) (24 hours arithmatic mean : 65 ${mu}g/m^3$, year average : 15 ${mu}g/m^3$). The percentage of $PM_{2.5}$ in $PM_{10}$ was 86.2% in platform, 81.7% in inside train, 80.2% in underground and 90.2% in ground. These results indicated that fine particles ($PM_{2.5}$) accounted for most of $PM_{10}$.