A study on the formation of particulate nitrate$(NO_3^-)$ and gaseous nitrate$(HNO_3)$ in the atmosphere was carried out in Seoul from Oct 8 to 11 1991. To collect $NO_3^-$ and $HNO_3$ in the ambient air, dual filter pack sampler (47mm$phi$) was used. In the dual filter pack sampler, the first filter was Teflon filter (poresize 1$mum$) for collection of $NO_3^-$ and the second filter was Nylon filter (poresize 0.45 $mum$) for $HNO_3$. Particulate nitrate$(NO_3^-)$ and Sulfate ions were analysed by Ion chromatography. $HNO_3$ concentration was higher in the day time $(9.93mug/m^3)$ than the night time$(3.50mug/m^3)$, and Particulate nitrate$(NO_3^-)$ concentration was higher in the night time and early morning$(6.21mug/m^3)$ than the day time$(4.31mug/m^3)$. The conversion rate of $NO_x$ to total nitrate$(NO_3^-, HNO_3)$ was 7.57%/hr in the day time and 4.79%/hr in the night time, and total average conversion rate was 5.60%/hr.