Abstract: In the carbon or oxygen isotope analyses of carbonate rocks, when thephosphoric acid (100%) is used to decompose the rocks and release CO_2, thereis one oxygne atom to be left from carbonate root and it will be transfered towater, this process is called as "mechanical isotopic fractionation" i. e. thefrationation between whole oxygen and 2/3 oxygen. It is necessary to correct this frationation effects according to concretecouditions because the fractionation processes differ under different reactiontemperatures and in different minerals. Usually using a simple approximatecalculation to realize the corrections: δ~(18)O_(M-CO3)=δ~(10)O_(M-CO2)+(Δ_1-Δ_M)this processing is, actually, to convert 2/3 oxygen isotope into whole oxygenisotope. If not all the standard samples and those to be examined are carbonaterocks, a complex aspect appears in mass spectrometric analyses. If only one ofthe samples (the standard sample or the sample to be examined) is not carbo-nate rock, the correction can be realized by the following equation: δ~(18)O_(X-PDB(CaCO_3))=(α-1)×10~3+α~(18)δO_(X-PDB(CO_2))If both of the samples mentioned above are carbonate rocks or not, we canexamine directly without any correction. In additin, the influences from other isotope fractionations should be eli-minate because only the molecular ion ratio such as 45/44 is measured but no~(12)C/~(13)C. The correction equation as above depond on the kind of receive plateof mass spectrometer and the standard sample. Therefore, we have to selectedifferent mass spectrometric method according to the different eases in practice.