The equation for estimatingαCO2 in metamorphic fluid of carbonate rocks is formulated by thermodynamic calculation for equilibrium state of decarbonation. The application of this method to marble from Jining Group in Inner Mongolia reveals that theαCO2 value of fluid in the progressive stage of granulite facies metamorphism is higher than 0.50,which is consistent with the data from recent researches. All evidence displays that theαCO2 value of fluid increased gradually in the progressive metamorphic process and reached 0.80 in its peak stage, which is attributed to the continuous separation of CO2with the intensification of decarbonation. Morever, some variations ofαCO2among samples (from 0.537~0.643) also suggest that CO2in the decarbonation process is most likely derived from internal buffering rather than from the deep source.