Abstract: Based on the linear increase in the amount of 231 Pa with time in a sample of highly enriched uranium (HEU), the HEU age is determined by measurement of the atomic ratio of ~231 Pa to ~235 U. The daughter nuclide ~231 Pa is separated and purified from its parent nuclide ~235 U by solvent extraction with N-benzoyl-N-phenylhydroxylamine (BPHA) with a recovery of higer than 67%. The radioactivity of ~ 231 Pa and ~235 U is measured with an HPGe γ-spectrometer and a large-area gridded ionization chamber α-spectrometer, respectively. The overall uncertainty in HEU age determination is estimated to be ca. 8%.