Abstract: A method was developed for determination of trace rare earth elements ( Sm, Eu, Gd and Dy ) in U-Nb mixed oxide using inductively coupled plasma mass spectrometry (ICP-MS). Though the contents of these four rare earth elements in nuclear fuel are below 10^-7g/g, the accurate determination of their contents in nuclear fuel is crucially important to the research of nuclear fuel for their thermal neutron-absorption cross-sections are large. After the sample was dissolved fast by HNO3-HF acids, U matrix was separated using CL-TBP extraction resin. ¹¹⁵In was used as internal standard to correct matrix interference and instrument drift for quantitative determination of Sm, Eu, Gd and Dy in eluate, semi-quantitative determination of U in eluate by ICP-MS. The method has a good linear correlation (r＞0.999) in the range of 0.01-1.00μg/L and the detection limit is between 0.001-0.002 μg/L. For the add concentrations of rare earth elements is 0.2 μg/L, the recovery is between 84.3%-97.3% and the relative standard deviation (s_r) is between 5.3%-8.7% （n=6）. The method can be available in practical application.