Abstract: An isobaric interference of CNO for boron isotopic measurement by negative thermal ionization mass spectrometry (NTIMS) has been studied. The result shows that the CNOis not only from the organic material, but also is from nitrate in loading reagent in NTIMS. Monitoring the mass 43 ion intensity and 43/42 ratio of blank are necessary for the boron isotopic measurement in NTIMS, other than boron content examining only.