The characteristics of one year observation aerosol data in Seoul, 200 I was studied using an OPC (Optical Particle Counter). The size resolved aerosol number concentrations of 0.3 ∼ 25 11m were measured. The results were compared with PM$_{10}$ mass concentration data under various meteorological conditions including dust and precipitation events. For fine particles whose diameter is less than 2.23 ${mu}{ extrm}{m}$, the number concentration increases in the early morning which is considered due to transportation. while the coarse mode particles increase during daytime. This increase can be explained as local sources and human activities near sampling site. Hourly averaged data show that there exists diurnal variation. Generally, PM$_{10}$ data showed a similar tendency with OPC data. The size resolved OPC data showed that the particles of 0.5 ∼ 3.67 ${mu}{ extrm}{m}$ are positively correlated with PM$_{10}$ data. The accumulated volume fraction of size resolved aerosol concentration in 0.5 ∼ 10 ${mu}{ extrm}{m}$ showed that 0.5 ∼ 2.23 ${mu}{ extrm}{m}$ particles occupied 59.2% of total aerosol volume of 0.5 ∼ 10 ${mu}{ extrm}{m}$./TEX>.