Abstract: Finnigan MAT 252 gas mass spectrometer has been used for oxygen and carbon isotope analysis, however, during the process of oxygen isotope analysis of carbonate minerals, the output results directly from the mass spectrometer would have a difference from the real δ 18 O value of carbonate minerals if the fraction( α 1) in the process of defining the isotope value of work gas is unequal to the fraction( α 2) during analyzing sample. In this paper we suggest a formulation to overcome the difference : δ x= δ 1+10 3ln( α 1/ α 2). After calibrated by the formulation, output results are compatible with the real value within error(±0.2‰) .