The dependence of the precision of uranium isotope ratio on particle diameter in secondary ion mass spectrometry (SIMS) was investigated. Each uranium particle in certified reference materials with different isotopic composition (U-235/U-238 = 0.01-1.00) was observed with a scanning electron microscope (SEM) and transferred by a manipulator on a glassy-carbon planchet for subsequent isotope ratio analysis with SIMS. The relative standard deviation (RSD) of uranium isotope ratios varied with particle diameter, the amount of less abundant uranium isotope and the number of measurement cycles. For the particle with the diameter less than 1.0 mu m, the RSD significantly increased with increasing the number of measurement cycles, which is due to the difficulty of obtaining constant signals for a long duration. In contrast, no significant increase in the RSD was observed in the analysis of the particle with the diameter larger than 2.0 mu m. As the results, it is estimated that the U-235 amount of 4.5 fg is sufficient to obtain the U-235/U-238 ratio with the RSD within 5.0%. For the analysis of minor isotopes, the U-234 amount of 0.42 fg and the U-236 amount of 1.1 fg are sufficient to obtain the U-234/U-238 and U-236/U-238 ratios with the RSD within 20%, respectively. (C) 2008 Elsevier B.V. All rights reserved.