Abstract: In this study, nitrate in standard samples, seawater and lake water and tap water samples were converted to N₂O using bacterial denitrification and chemical conversion, the nitrogen and oxygen isotope composition were determined by Isotope Ratio Mass Spectrometry (IRMS), and the parameters of the nitrogen and oxygen isootope calibration curve, the precision, accuracy and detection limit of the two conversion methods were obtained. The results show that: (1) The yield rates of nitrate converting to N₂O were all greater than 90%. (2) The slope of the nitrogen isotope calibration curve for the bacterial denitrification and the chemical conversion were 0.959 and 0.481, respectively, Close to the respective theoretical values of 1 and 0.5; The slope of the oxygen isotope calibration curve were 0.908 and 0.697, respectively, there is a certain deviation from the theoretical value of 1. (3) The standard deviations of nitrogen isotopes determined by the two methods were less than 0.5‰, The standard deviations of oxygen isotopes were all less than 1‰. (4) The detection limit of nitrate nitrogen and oxygen isotope test by bacterial denitrification and chemical conversion is 4.4 nmol and 7.54 nmol, respectively. (5) Both methods can more accurately determine the nitrogen isotopic composition of nitrate in the three types of samples (sea water, lake water and tap water). The results were consistent by the two methods. The difference of nitrogen isotope results by the two methods were between 0.11‰ and 0.77‰. The oxygen isotope difference were from 3.67‰ to 5.75‰, It was shown that oxygen isotopic fractionation were occur when the nitrate were reduced to N₂O, and the degree of oxygen isotopic fractionation were difference in the two conversion methods.