Abstract: Isotope dilution combining with multi-collector inductively coupled plasma mass spectrometry through measuring ²³⁵U/²³¹Pa to determine the age of high enriched uranium(HEU) was studied. The spike of ²³³Pa was separated from ²³⁷Np after twice TTA extraction and back-extraction. The decontamination factor(DF)(Np) in ²³³Pa exceeds 200. The concentration of ²³⁵U and ²³¹Pa was measured by isotope dilution mass spectrometry using spike of ²³³U and ²³³Pa which calibrated in-house by the reference material CRM U900, respectively. So the ratio of ²³⁵U and ²³¹Pa can be calculated, and HEU age can be obtained. The method was used to determine the age of the reference material CRM U850, and the relative error of results are 1.97%. The method can be used to measure HEU age in nuclear forensics and nuclear safeguard.