The behavior of persistent organic pollutants in soil environment largely depends on their adsorption to soil organic matter.Particle-size fract ionation is usually used to differentiate organic matter pools with different composition and turnover rates.In this research,phenanthrene (Phe) and benzo[a] pyrene (Bap) were used as representatives of low-and high-ring polycyclic aromatic hydrocarbons (PAHs),respectively.The distribution characteristics of them in different particle-size separates (clay,fine silt,coarse silt,fine sand and coarse sand) of nine agricultural topsoils (0~20 cm) from a contaminated district were studied.The results showed that the average contents of Phe in different particle-size separates decreased in the order coarse sand>fine sand>clay>fine silt>coarse silt,and those of Bap in the order coarse sand>fine sand>coarse silt>fine silt>clay.In the different particle-size separates,the contents of both Phe and Bap had significantly positive correlations with that of organic matter (p<0.01).The enrichment ability of organic matter for Phe in different particle-size separates decreased in the order coarse silt>fine silt>fine sand>coarse sand>clay,and for Bap in the order coarse silt>coarse sand>fine silt>fine sand>clay.