Abstract: The factors effecting ionization and transmission in measuring isotopeabundance ratio (~(235)U/~(238)U) of uranium by MAT-260 mass spectrometer aredescribed. The fractional effect and the influence of focusing potential wasmeasured for 10 μg, 1μg, 0.2 μg sample. The instrument was calibrated by thestandard sample NBS-010 and the curve of calibration was made. The preci-sion is better than 0.15% for natual and middle abundance urauium.