Abstract: The uncertainty sources in determination of fluorine content of cryolite and aluminium fluoride by distillation-thorium nitrate volumetric method were analyzed.Evaluation for the uncertainty of each component was carried out by establishing the mathematic model，and the expanded uncertainty of fluorine content determination with distillation-thorium nitrate volumetric method was also calculated.Experimental principle：melt the sample with anhydrous sodium carbonate，separate fluoride through sulfuric acid-water vapor distillation，taking sodium alizarin sulfonate and methylene blue as indicators，using thorium nitrate standard solution to titrate fluorine content in the sample.Evaluation results indicated that，the expanded uncertainty of thorium nitrate volumetric method for the determination of fluorine content was 0.42%.

Key words: distillation, volumetric method, fluorine content, uncertainty