Abstract: Objective: To evaluate the uncertainty of determination of nine pesticides in tea by gas chromatography and mass spectrometry (GC-MS). Methods: The uncertainties of nine pesticide residues in tea were evaluated by GC-MS combined with internal standard quantitative method. Five evaluation components, including pesticide standards preparation, sample weighing, extraction, purification and detection were studied. Results: The main uncertainty in the determination of pesticides came from the fitting process of standard curve (u_rel(R)=1.45%~7.93%), sample recovery (u_rel(Q)^a=2.64%~3.54%, u_rel(Q)^b=1.62%~5.89%) and standard solution preparation (u_rel=2.46%), then the repetition of the sample measurement (u_rel(A)=0.5%~2.56%), especially at the extremely low concentration. Conclusion: For the reliable results, it is important to choose the accurate sample preparation, the concentration range of standard curve. And when the results indicates positive, reassays should be done in triplicate.