Abstract: When more than two elements on the filament of a thermal ionization mass spectrometer (TIMS) are vaporized, the element ratios in the vapor depend on the filament temperature. The ion current ratios of different mass numbers vary with the temperature, which is dominated by the filament current. With the presence of isobaric interference, the method of isotopic analysis by TIMS is established in the present paper and applied to determine the natural lutetium abundance in the solutions, in which ytterbium and lutetium are mixed by ratio of 1 000∶1. The measured isotope ratio of lutetium is in good agreement with the standard value. Likewise, the concentrations of ¹⁷³Lu and ¹⁷⁴Lu in the real samples produced through bombardment of ytterbium target by proton beam are determined by ID-TIMS. The relative uncertainties of concentrations are 0.45% and 1.1%, respectively. 