Abstract: With H₃PO₄ as the ionization intensifier, the isotope ratios of uranium in the depleted uranium samples with uranium concentration at nanogram-level were determined by thermal ionization mass spectrometry (TIMS) with the precision of ＜2.9％RSD. With ²³³U as the diluent, the concentration of uranium in the samples were also determined by isotope dilution mass spectrometry (IDMS) with the expanded uncertainty of 2.4% .The research results showed that in uranium isotope ratio determination for the samples with nanogram-level of uranium, uranium background interference from rhenium filament and other sources should not be ignored, which is needed for further study to remove the background interference.