A colourimetric method using 96-well microplate techniques for the determination of NO^3--N in water samples is described.Nitrate was determined by azo dye formation after reduction to NO₂-using a solution of hydrazine sulphate.The recovery rates of NO^3--N were in the range of 88.0%～109.8%.The limit of quantitation of the colourimetic assays was 0.015 mg/L.The NO^3--N values measured by the colorimetric method with microplate reader were correlated with those by continuous flow analysis.The correlation coefficient was R = 0.987 **(n=29，P<0.01)and linear slope was near 1.0.The relative standard deviation(RSD)of proposed colorimetric method with micro-plate reader was less than 5%(n=6).It is suggested that the hydrazine sulphate reduction and azo dye colorimetric assay with a standard 96-well microplate format is readily applicable and can be used for nitrate rapid monitoring in water samples.