Since the special water storage regime in Three Gorge Reservoir, the periodic rise and fall of water table take place within 30m’s elevation difference. So, the different hydrological sections and water level elevations in the water level fluctuating zone of Yangtze River tributaries undergo a different dry-wet alternation. The dry-wet alternation make variable hydrological processes, regional climate, vegetation adaptability and particle size composition of sediment change obviously, which may transfer the role of sediment instead of a nitrogen sink into nitrogen source. The determined total nitrogen (TN) and inorganic nitrogen, such as ammonium-nitrogen (NH₄⁺+-N) and nitrate-nitrogen (NO₃^--N) could reflect background level of nitrogen in sediment, whereas the measured total transformable nitrogen (TF-N), weakly bonded with sediment lattice, was a good indicator for nitrogen cycle. Sediment is composed by different size particles. Since the larger particles are hardly broken down into small size for releasing the dissolved nitrogen into water, the absorbed nitrogen on the surface of the particles and the smaller particles would easily release into water. Frequent dry-wet alternation is a crucial factor affecting soil particle size composition. Sediment plays a key role as a source and pool in nitrogen recycling in water body. So it is important to understand the relationship between sediment particle size composition and nitrogen distribution in the water level fluctuating zone as affected by dry-wet alternation in elaborating mechanism of eutrophication of the water body in tributaries of the Three-Gorge Reservoir. The Pengxi River is a typical tributary of the Yangtze River in the Three-Gorge Reservoir Area. The water level fluctuating zone of the river was selected for the study. Sediment samples were collected, 5 each sampling site, different in depth, at two water level elevations and the sampling sites were distributed at three hydrologic sections of the river, upper, middle and down streams of the river. Particle size composition, TN, TF-N, NH₄⁺-N, NO₃^--N in the sediment samples were determined. Relationships of sediment particle size composition with content and distribution of TN, TF-N, NH₄⁺-N and NO₃^--N were analyzed with the sediment fractal dimension (D_m) theory. Results show that the sediment was composed of sand, silt and clay, accounting for 49.75%, 36.16% and 14.09%, respectively, in the water level fluctuating zone of the Pengxi River. D_m varied in the range of 2.612~2.781, being 2.727 on average. With water level elevation declining, position of the hydrologic section moving downward along the river and sampling going down in depth, the proportions of clay and silt and D_mdecreased and the proportion of sand increased in the sediment. The contents and distributions of TN, TF-N and NO₃^--N were found to be positively related to the proportions of clay and silt in and D_mof the sediment, but negatively to the proportion of sand in the sediment. D_m may serve as an important indicator of sediment structure for evaluating eutrophication risk of the tributary in the Three Gorge Reservoir Area. Generally speaking the sediment in the region is good in textural structure, but the frequent dry-wet alternations decreased the water and nutrient retention capacity of the sediment, while making the sediment sandy in the down stream of the river low in water level elevation, which in turn affected N distribution. TF-N in the sediment in the upper stream during the dry season and in the top layer under high water level elevation increased, thus becoming a source contributing to the eutrophication risk of the water body during the wet season.