Abstract: A method for the evaluation of the uncertainty of clenbuterol residues in meat products by UPLC-MS/ MS was established. The influencing factors on the uncertainty in the procedure of the content determination were evaluated, including repeatability of results of measurements, the fitting of standard curves, the preparation of standard solutions and sample solutions and the analytical instrument.According to JJF1059-1999, the component uncertainties were synthesized and extended.An expended uncertainty of 16.08% was obtained at the confidence probability of 95% and coverage factor of k = 2.Based on the analysis of uncertainty, the fitting residual of standard curves were the major source of the uncertainty. Meanwhile, some strategies for improving quality control were proposed.Measurement of uncertainty was applicable to set the limit of the clenbuterol. It was important to the standards studies of uncertainty of clenbuterol residues in meat products.