Objective: The uncertainty of determination of three pyrethroid pesticides （cypermethrin, fenvalerate and deltamethrin） in fresh tea leaves by gas chromatography was evaluated. Methods: A mathematical model was established according to the determination process of the experiment, and the uncertainties caused by various factors in the model were identified. The sources are mainly the following 7 aspects: repeated determination of samples, fitness of standard curve, recovery rate, standard solution preparation, sample weighing and pretreatment, and the measurement of instrument. Results: At the spiking level of 0.10 mg/kg, the expanded uncertainty of measured values of cypermethrin, fenvalerate, and deltamethrin were 0.011, 0.011 and 0.012 mg/kg, respectively. The detection results of three pesticides in fresh tea leaves in 95% confidence interval could be expressed as cypermethrin: （0.093±0.011） mg/kg, fenvalerate: （0.094±0.011） mg/kg, deltamethrin: （0.098±0.012） mg/kg （k=2）. Conclusion: The uncertainties of the method were mainly introduced by standard solution preparation, calibration curve fitting and recovery rate, indicating that these aspects should be controlled strictly in the following sample analysis.