Abstract: The error transmission of the determination values of polyisotopic element--uranium by isotope dilution mass spectrometry is discussed.The error trans-mission factor related to the forms of isotope abundance ratios in basic for-mula is described. It is suggested that the ratio of the sample to tracer in aparticular case may be selected according to the actual analytical conditions.This provides reference for selecting the proper instrument and experimentalconditions for analyzing the polyisotopic elements with more than two isoto-pes.