We studied an analytical method for 4 organic acids will be regulated in 2010 using on-line thermal desorber with gas chromatograph/flame ionization detector. Results for each compounds showed good linearity(r² ＞ 0.99) and good precision(RSD ＜ 3%). Minimum detection limit values are about 2~3ppb when we sampled 1.5 L. These values will be reduced to 0.4~0.5 ppb when sampling 10L. We analyzed the 56 ozone precursor standard gas using the same method to see if there are any peaks to be overlapped in ambient air and the results showed that there is no peak overlapped. The linearity, precision and MDL in this study satisfied the guideline of Korean standard method for 4 organic acids. This analytical method in this study could be utilized effectively as on-line monitoring instrument to detect 4 organic acids.