Abstract: The objective of this paper was to investigate and quantify the main form of N loss by runoff, and finally to serve for the N management and loss simulation. Three agro-watersheds with different land use type and terrain were taken examples to investigate the main form of N loss by runoff under erosion of natural rainfalls in purple soil region. Results showed that nitrate nitrogen was the main form of N loss by runoff. Generally, nitrate nitrogen concentration in runoff accounted for 80 percent of N loss, even above 90 percent in farmland-dominated watersheds. In addition, the average concentration of N loss was (5.36±2.13) mg/l in farmland-dominated watersheds, while behaving from 1.70 to 3.15 mg/l in woodland-dominated watersheds and watersheds with various landscape structures. This showed that precise management of nitrate nitrogen in farmland-dominated watersheds was a key practice for control and reduction of agricultural non-point source N loss.