The study discussed the source of uncertainty about measuring the Al₂O₃ of bauxite by X-ray Fluo-rescence Spectrometry in detail; moreover, we assessed the uncertainty of all components possibly. According to the research we could know there were several reasons that caused the uncertainty about measuring the Al₂O₃ by X-ray Fluorescence Spectrometry, such as the method of measurement, standard reference materials, weighing error, etc. The uncertainty of each compo-nent was calculated and analyzed quantitatively; the extended uncertainty of measurement results was obtained from synthesis of each uncertainty source.