Abstract: Highly enriched isotopic reagents,~(151)Eu_2O_3 and ~(153)Eu_2O_3, of definite chemical purity were used to calibrate a solid ion-source thermal ionization mass spectrometer to find the mass discrimination effect, which applied to correct the isotopic abundance ratio measured for two minerals. Then, together from the known nuclidic masses the atomic weight of europium so calculated is 151. 964±0.003, agreeing well with the current value.