Abstract: In order to improve the accuracy of test results, the uncertainty in determination of quinclorac in soil with HPLC was systematically calculated according to JJF 1059-1999Evaluation and expression of uncertainty in measurement. The results indicated that:1) Preparation of standard working solution, calibration of working curve and constant volume of sample solution were the major factors influencing the uncertainty; 2) The combined uncertainty was 0.00069 mg/kg and the expanded uncertainty was 0.0014 mg/kg (level of confidence of 95%, k=2) at the concentration of quinclorac in soil of 0.0236 mg/kg. The uncertainty in determination of quinclorac in soil with HPLC could be decreased by using high purity standard substance and qualified metering devices, and strictly controlling treatment procedures.