Abstract: Uncertainties originating from the measurement of H₂S content in natural gas by iodometric titration method were evaluated.The uncertainties were mainly obtained from four sources:the calibration of sodium thiosulfate standard solution, bubble absorption method for H₂S, titrated analysis of absorption liquid and repetitive measurement.From results of uncertainty analyses, the uncertainty values of four factors were 0.082%, 0.16%, 0.17% and 0.71%~7.07% respectively.When the concentration of H₂S in sample was 12.5 mg/m³, the designed confidence level was 95% and converage factor(k) was 2, the relative expanded uncertainty(U) of the H₂S concentration was 7.08%.