Abstract: The nitrogen balance of agro-ecosystem in a typical watershed in the hilly area of central Sichuan Basin was studied. The results show that there is an excess application of nitrogen which is 363.5kg／hm² in dryland and 161.8kg／hm² in paddy field, respectively. The chemical nitrogen accounts for 76.65% of the total nitrogen input to the agro-ecosystem. Besides crops' harvest, nitrogen loss from agro-ecosystem amounts to 129.67kg／hm²·ɑ mainly by gas. Runoff, leaching and soil erosion are other important losses primarily from slope cropland which averages 44.34kg／hm²·ɑ. The nitrogen surplus of agro-ecosystem in the small watershed is 66.8kg／hm²·ɑ, which increases the nitrogen loss and results in non-point source pollution of nitrogen to some extent in this watershed.