Abstract: Some problems of determining uranium isotope abundance by MAT-260 mass tpectrometer, especially, the influence factors on precision and accuracy are di-acussed. NBS standard samples are used for calibrating the acquisited data. Onthis base, natural uranium, uranium of isotope abundancc 50% as well as deple-ted materials are determined for many times. Measured precision of R_(58) (abun-dance ratio of ~(235)U to ~(238)U) is better than ±0.06%, accuracy within ±0.2%.