We developed a method for high-sensitivity nitrogen isotopic analysis of particulate organic nitrogen (PON) in seawater and freshwater,for the aim of measuring the rate of nitrogen fixation through 15 N2 tracer technique for seawater samples that contain low numeric abundance of organisms.The method composed of the traditional oxidation/reduction methods: such as oxidation of PON to NO3 - using persulfate,the reduction of NO3 - to NO2 - using spongy cadmium,and further reduction of NO2 - to N2O using sodium azide in an acetic acid buffer.Then,N2O is purged from the water and trapped cryogenically with subsequent release into a gas chromatography column to analyze stable nitrogen isotopic compositions (δ 15 N) using a continuous-flow isotope ratio mass spectrometer.A standard deviation less than 0.3 ‰for nitrogen was found for PON samples containing more than 80 nmolN,with a blank nitrogen of 20 nmolN.Nitrogen isotopic fractionation were consistent within each batch of analysis.Besides,we found that we can also determine nitrogen isotopic compositions (δ 15 N) of total dissolved nitrogen (TDN,sum of NO3,NO2 -,ammonium,and DON),by applying the method for filtrate samples.Our method may prove to obtain isotopic information for lower quantities of PON and TDN than other methods,so that we can reduce quantities of water samples needed for incubation to determine the nitrogen fixation rate in both seawater and freshwater.Besides,the method is also useful to determine the rate of nitrogen fixation by the small size fraction in ocean.By using the method,we determined the nitrogen fixation rate for surface seawater samples taken at western subtropical North Pacific area.Besides to the nitrogen fixation rate,we also quantified both mixing ratios and production rates of H2 for the same seawater samples.We found that the nitrogen fixation organism play significant role for the H2 production in the studied area.