Abstract: In the application of UV-spectrophotometry to the determination of NO-3 in water (i.e.,the cawse method),it was found that better sensitivity and accuracy was well as selectivity were obtained by measuring the absorbance at the wavelength 200 nm than at 210 nm which was used in the original procedure.By modification of changing the determination wavelength to 200 nm,linear relationship between absorbance and concentration of NO-3 ion was obtained in the range of 0.05-3.00 mg·L-1 as shown by the linear regression equation y=0.006 6+0.715 5 x (r=0.999 8).Molar absorptivity of the proposed method was found to be 1.02×104L·mol-1·cm-1.It was found that NO-2 (<1.0 mg·L-1) and Cl- (<200 mg·L-1) were tolerable in the determination of NO-3 in different water samples.