Abstract: Xishuangbanna, located in southwest China, bounded by Laos and Myanmar on the upper Mekong River, is on the northernmost edge of tropical Asia. As a result, a mosaic distribution of tropical seasonal rain forest, which mainly appears in the wet valleys, usually with small streams or on the low hills and flats below 1000 m altitude exists in Xishuangbanna. The tropical seasonal rain forest in Xishuangbanna is rich in species and has a complex forest canopy and structure. In 1999, 119 species with DBH>10 cm were recorded in the 1 hm2 study plot. Based on the “small catchment” approach and input-output budgets, we studied on the nitrogen cycling in the seasonal rain forest. The nitrogen stock in the seasonal rain forest ecosystem (6 481.2 kg·hm-2) was higher than that of montane rain forest in Hainan of China (6 345 kg·hm-2), but lower than montane rain forest in New Guinea (20 190 kg·hm-2), lowland rain forest in Brazil (7 537 kg·hm-2), and lowland rain forest in Ghana (7 230 kg·hm-2). The distribution of the nitrogen stock in the seasonal rain forest ecosystem is as follows: 970.9 kg·hm-2 (15.0%) in the living biomass, 37.7 kg· hm-2 (0.6%) in the forest floor, and 5 481.2 kg·hm-2 (84.4%) in the soil (0-30 cm). We found that most of the nitrogen in the seasonal rain forest was stored in the soil, but not in the living biomass. Soil (0-10 cm) net nitrogen mineralization rate was 90.4 kg·hm-2·a-1. Soil played a very important role of stocking and providing N to plants. The nitrogen content in precipitation, throughfall, stemflow and stream flow were 0.565, 0.828, 0.983 and 1.042 mg·dm-3, respectively. The nitrogen input from precipitation, throughfall and stemflow were 8.89, 10.97 and 3.57 kg·hm-2·a-1, respectively. The nitrogen output in stream flow was 5.95 kg·hm-2·a-1. The nitrogen net increase in the seasonal rain forest was 2.94 kg·hm-2·a-1. In the nitrogen biological cycling, 149.86 kg·hm-2·a-1 was taken up by plants，69.30 kg·hm-2·a-1 stored in living biomass，and 80.56 kg·hm-2·a-1 returned to soil. This undisturbed seasonal rain forest had the nitrogen conservation mechanism and remained in the nitrogen accumulation state.