Abstract: In a DTPA-Tris buffer medium of pH 8.2, NO-3 was reduced to NO-2 by UV irradiation reduction, and a colored product with its absorption maximum at the wavelength of 540 nm was formed by its reaction with the color reagent. Based on these facts, a FI-spectrophotometric method with on-line UV irradiation reduction for determination of NO-3 in seawater was proposed. 30 g·L-1 NaCl solution was used as carrier. The lengths of reaction coil and sampling ring chosen were 180 mm and 130 mm respectively. Linearity was kept between values of mass concentration of NO-3 and the corresponding absorbances in the range from 9×10-3 to 3.0 mg·L-1, with the detection limit (3s/k) of 4.63 μg·L-1. In analyzing a natural seawater sample for 10 determinations, an average concentration of NO-3 of 0.538 mg·L-1 was obtained with RSD of 0.34%. The reduction rate of NO-3 was found to be 80%. Recovery was tested by addition of 0.50 mg·L-1 of NO-3 standard solutions to each of 6 seawater samples, values of recovery found were more than 97%.