As the establishment of a programme for evaluation of the uncertainty of rapid analytical method is claimed in Guide to the Expression of Uncertainty in Measurement ,the evaluation of the uncertainty of Pb measurement in calcined magnesite by GFAAS was proposed in this paper. The sources of error and its effects on the total uncertainty were analyzed. The uncertainty of results were evaluated by statistics. The collecting of various parameters and methods of calculation were illustrated. The combined standard uncertainty and the expanded uncertainty were reported .