The aerosol number concentration have measured with an aerodynamic particle sizer spectrometer(APS) at Gosan site, which is known as background area in Korea, from January to September 2011. The temporal variation and the size distribution of aerosol number concentration have been investigated. The entire averaged aerosol number concentration in the size range 0.25~32.0 ${mu}m$ is about 252 particles/$cm^3$. The number concentration in small size ranges(${leq}0.5{mu}m$) are very higher than those in large size ranges, such as, the number concentration in range of larger than 6.5 ${mu}m$ are almost zero particles/$cm^3$. The contributions of the number concentration to PM10 and/or PM2.5 are about 34%, 20.1% and 20.4% in the size range 0.25~0.28 ${mu}m$, 0.28~0.30 ${mu}m$ and 0.30~0.35 ${mu}m$, respectively, however, the contributions are below 1% in range of larger than 0.58 ${mu}m$. The monthly variations in the number concentration in smaller size range(<1.0 ${mu}m$) are evidently different from the variations in range of larger than 1.0 ${mu}m$, but the variations are appeared similar patterns in smaller size range(<1.0 ${mu}m$), also the variations in range of larger than 1.0 ${mu}m$ are similar too. The diurnal variations in the number concentration for smaller particle(<1.0 ${mu}m$) are not much, but the variations for larger particle are very evident. Size-fractioned aerosol number concentrations are dramatically decreased with increased particle size. The monthly differences in the size-fractioned number concentrations for smaller size range(<0.7 ${mu}m$) are not observed, however, the remarkable monthly differences are observed for larger size than 0.7 ${mu}m$.