Abstract: 17 free amino acids, in corn steep liquor were detected by amino acid analyzer according to GB /T 18246-2000. The “bottom-up ” approach was used to estimate the sources of uncertainty, such as repeatability, weighing of the sample, sample volume, sample dilution, stock solution and molar mass. The range of expanded uncertainties for all of the free amino acids was 0.01% to 0.04%. Taking glutamic acid as an example, the percentage contribution of each source to the relative combined uncertainties was calculated.The data of glutamic acid indicated that the major contributions to the uncertainty budget originating from the stock solution and repeatability.