Abstract: The measured accuracy of magnesium and germanium isotope ratios by electron multiplier is limited by biases related to the differences in mass and intensity of the ion beams being measured. The accuracy is also subjected to the error resource from mass fractionation. The methods and general equations corrected for the biases in the isotopic ratios measured by electron multiplier were given in this paper. The methods and equations corrected for the mass fractionation by normalizind method based on the correlative line between isotopic ratio and ratio were also described. Finally. the results obtained from nature-magnesium isotopic ratios and nature-germanium abundances are ~(26)Mg/~(24)Mg=0.13938, ~(25)Mg/~(24)Mg=0.12660; ~(70)Ge=20.50, ~(72)Ge=27.48, ~(73)Ge=7.76, ~(74)Ge=36.51, and ~(76)Ge=7.75, respectively.