Abstract: The trace sulfur hexafluoride in high pure nitrogen trifluoride gas was determin by selective ion monitor (SIM) mode. The analysis is carried out on a poraPLOT Q capillary column (5 μm×0.32 mm×30 m) and QP5000 GC/MS. The quantitative selected ion is m/z 127. The interferece to the determination of selected ion was studied. The linear range is from 2.0 to 100 μL/L with relative coefficient of 0.9996. The mininum detection limit is 0.2 μL/L. The average recovery is 92.70%~97.04%. The intra-day and inter-day relative standard deviation(s r) of SF 6 is less than 4.17% and 5.33%, respectively.