This paper focused on analytical validation such as the amount needed, time drift, repeatability, memory effect, analytical precision and accuracy on carbon/oxygen isotopic measurement of small carbonate samples (10-80 A mu g) using an IsoPrime100 mass spectrometer. 10 A mu g of carbonate samples yielded mass 44-intensity of 2 nA. The minimum amount needed was 10 mu g and the repeatability was around 0.1 aEuro degrees (1 s.d.). Through 24-h measurement there was no temporal variation of delta C-13, but that of delta O-18 showed variation with decreasing the amount of CO2 reference gas, suggesting that delta O-18 values need to conduct time drift correction. Inter-sample memory effect was not observed, indicating that IsoPrime100 mass spectrometer with CO2 purification system is adequately evacuated after each analysis by means of pumping system. Moreover, IAEA-CO-1 and JAMSTEC-WS values calibrated by international reference NBS19 agree within error with the reference values, demonstrating the accuracy of our analytical procedure on carbon/oxygen isotopic measurement of carbonate samples. This paper offers insights into analytical validation procedure when setting up a mass spectrometer on isotopic analyses of small carbonate samples.