Abstract: The isotopic ratio of iridium is a useful tool for the identification of burn-up of nuclear fuel in nuclear industry. The key issue for precise measurement of isotopic ratios of iridium in trace amount is to improve ionization efficiency, in order to generate sufficient ion intensity and stable ion beam. Negative thermal ionization mass spectrometry (N-TIMS) is proven to be a powerful method for the analysis of nuclides with relatively high ionization potentials. In this work, a novel method was established based on N-TIMS for the precise measurement of Ir isotopes. The optimized condition was developed after comparing IrO₂^- ion beams under different circumstances. The ionization efficiency of 50 ng Ir could reach up to 9.8×10^-4 when employing Pt single filament and Ba(OH)₂ as ionization activator. The iridium isotope ratios of n(¹⁹¹Ir)/n(¹⁹³Ir) obtained after oxygen isotopic correction agree well with the certified values in the range of the analysis error, and the external precision for 10 ng Ir sample loads is better than 0.2%, which can fulfill the requirements for applications in nuclear industry.