Abstract: The boron isotopic composition of NIST 951 boric acid has been determinated by positive thermal ionization mass spectrometry based on the measurement of the Cs 2BO + 2 ion with coating graphite on the filament substrate. In the meantime, the effect of the loading quantity ranging from 100 ng to 10 ng B on the boron isotopic ratios and precision has also been compared. The results show that the best precision is acquired by loading more than 100 ng B and the lowest detection limit is 10 ng B loaded using a Faraday cup detector. This method is helpful for the determination of isotopic composition of lower content boron (nano-gram level) in the geological samples.