Abstract: Carbon and nitrogen isotopes are essential tools to study paleoclimate, palaeoenvironment, and ecosystem. At present, carbon and nitrogen isotopes are commonly determined by Elemental Analyzer-Isotope Ratio Mass Spectrometer (EA-IRMS). Reaction temperature and sampling weight directly affect the burning of samples, and thus affect analytical precision. Using EA-IRMS technology and taking standard samples as references, the carbon and nitrogen isotope values were determined at different conversion temperatures to study the optimum reaction temperature, in order to ensure the accuracy of the analysis presented in this paper. At the same time, by analyzing the detection limits of samples with different nitrogen contents, the relationship between the nitrogen content of the sample and the lowest detection limit was determined and thus the lowest quantity of samples for accurate determination of nitrogen isotopes were also defined. The results show that reaction temperature has a significant effect on analytical precision. Analytical precision of carbon isotope is less than ±0.2‰ when the reaction temperature is either 900℃ or higher than 900℃, but the precision of nitrogen isotope can reach ±0.3‰ only when the reaction temperature is no lower than 950℃. The linear relationship between nitrogen content and detection limit was expressed as R²=0.873 according to the data. According to this relationship, the sample introduction quantity can be determined and controlled when analyzing nitrogen isotope.