Objective: The uncertainty in determination of sulfamonomethoxine（SMM）, sulfadimethoxine（SDM）, sulfamethoxazole（SMZ） and sulfamethazine（SM₂） in pork was evaluated by using LC-MS/MS techniqne. Methods: The uncertainty caused by various factors in the whole process of determination was analyzed, including weighing, solution preparation, sample extraction and purification, apparatus measure，recovery，etc. Results: The expand uncertainties for SMM, SDM, SMZ and SM2 were 9.4, 18.1, 12.0 and 15.6 μg/kg, respectively. Conclusion: The results showed that the main sources of uncertainty arouse from the measurement repeatability, standard solution and curve fitting.