Abstract: Cobalt in alloy of 1YC8 was determined potentiometrically with an Auto-titrator.Based on a comprehensive understanding of the whole procedure of the determination,mathematical model for evaluation of uncertainty in measurement was constructed and the sources of uncertainty and their effects on the total uncertainty were analyzed.On the base of the evaluated uncertainties of A type and/or B type,the combined standard uncertainty and finally the expanded or total uncertainty of the method were given.Result of cobalt in the alloy determined by the automatic potentiometric titration with its confidence interval was given as 10.92%±0.07%.