Abstract: The measurement uncertainty of the analytical results for carbonate in natural gypsum samples by acid base titration method was evaluated and the mathematic model for uncertainty expression was established. The total uncertainty mainly came from the sub-uncertainties of sample preparing process, titration process, calibration of NaOH and HCl titrants, repetitive experiments, reagent blanks, etc. The extended uncertainty for the method was obtained from the combined uncertainty multiplied by extended factor of 2. When the average content of CO2 in the samples was 8.66%, the expanded uncertainty was 0.17%.