Abstract: Carbon nanotubes (CNTs) were used as the ion emission promoter for increasing the ionization yield of uranium for the measurement of uranium isotopes by thermal ionization mass spectrometry (TIMS). The emission effect of the carbon nanotubes was studied by researching the change of ion intensity, signal stability and measurement precision. A comparative study of the emission performances of graphite, sucrose and three kinds of carbon nanotubes with different diameters was carried out. The results suggest that carbon nanotubes loaded with the uranium sample can significantly improve U⁺ yield, signal stability and measurement precision. The ion emission capacity of CNTs-3 is the best among all the carbon sources investigated in this work. The ion yield of U⁺ is increased more than three times when CNTs-3 as ion emission promoter. Samples with 25 ng and 5 ng uranium are measured using CNTs-3 as ion emission promoter. The results show that the relative standard deviations (RSD) of ²³⁵U/²³⁸U is about 0.2% and 0.5% (n=18), respectively.