Abstract: This paper expressed the methods of determination of trace impurities in gases by Atmospheric Pressure Ionization Mass Spectrometer (API-MS). The trace impurities N 2O and NO in high purity nitrogen gas were determined quantitatively by API-MS, and the total uncertainty of the method was 15% and 17%, respectively. The analysis conditions and effects with determination of N 2O and NO by API-MS were studied and discussed.