Abstract: A method of determination of trace Dy in U3O8 by isotope dilution mass spectrometry (IDMS) is described. The quantity of sample is Ig U3O8, 160Dy is used as spike. The great quantity U and trace elements Fe and Pb are removed by model 201×7 anion ion-exchange resin. Dy is separated from other rare earth elements by P507 extractant-containing resin. The ratio of 160Dy/162Dy is measured by mass spectrometer and the quantity of Dy is calculated. The precision of method is 4. 1% as sample contains 8. 47×10-8Dy.