Abstract: In order to estimate amounts of the ammonium nitrogen adsorbed by different fractions of natural organic matter in sediment, effects of fractions of natural organic matter in sediment on sorption of ammonium nitrogen were investigated by batch experiments of equilibrium adsorption. Results show that sorption capacity of the sediment reduced dramatically after its organic matter was removed, with normalized sorption capacity and partition coefficient equaling only to 55.30% and 69.49%, respectively, that of the heavy fraction of organic matter, indicating that organic matter is a main factor affecting sorption of ammonium nitrogen on sediment. However, sorption of ammonium nitrogen in the light fraction of organic matter was dominated by distribution (K_oc=85.57). Stably combined humus and tightly combined humus were important adhesive substances in the formation of loose and porous aggregate structure of the sediment. In the heavy fraction of organic matter, besides the mechanism of distribution, sorption of ammonium nitrogen was controlled by the mechanism of pore-filling, (Г_moc=3 477.81 mg·kg^-1).Furthermore, tightly combined humus (humin) played a leading role in sorption of ammonium nitrogen, (Г_moc=5 857.78 mg·kg^-1). Consequently, in estimating amount of the ammonium nitrogen adsorbed by the light fraction of organic matter, stably combined humus and tightly combined humus, it is advisable to count it as 0.32, 1.21 and 1.68 times, respectively, of that by the heavy fraction of organic matter.