Effects of low-molecular-weight organic acids (LMWOAs) on residues and forms of phenanthrene and pyrene in yellow brown earth was investigated using a microcosm testing procedure. It was found that after 60 days of aging, the total residual concentrations of phenanthrene and pyrene in the soil decreased significantly to only 10.13 and 29.18 mg kg^-1, respectively, showing a removal rate of 87.33% and 63.50% as compared with the control without the addition of LMWOAs. The test PAHs increased in residual concentration, but decreased in degradation rate with 0~64 mg kg^-1of citric acid, oxalic acid or tartaric acid added. Further investigations revealed that a small amount (≤4 mg kg^-1) of organic acids was adequate to inhibit significantly degradation of PAHs in the soil. The degradation of test PAHs in the soil depended markedly on microbial activities in the soil environment. As pyrene is higher in molecular weight and complexity of structure, pyrene is more resistant to microbial degradation than phenanthrene in the soil. The PAHs in soil could be fractionated into desorbing fraction, non-desorbing fraction, and bound residue. The former two fractions were observed to be the dominant forms of the test PAHs in the soil irrespective of addition of LMWOAs. However, the addition of LMWOAs increased the concentrations and proportions of the two forms of the PAHs in the soil. It did the desorbing fractions of phenanthrene and pyrene by 46.67%~749.1% and 1.83%~80.20%, respectively and the non-desorbing fractions by 8.73%~375.2% and 22.63%~114.3%, respectively. The fractions of bound residue of the PAHs remained to be the smallest in the soil irrespective of whether organic acids were added or not.