Abstract: An evaluation method for uncertainty in measuring ochratoxin A(OTA) in cereal grains with HPLC was established.Standard uncertainty and expanded uncertainty of the components in the process of the determination were calculated,an evaluation and comparison on the sources of uncertainty and the relative contribution were made. The results showed that the sources of the determination result uncertainty could be attributed to uncertainty of calibration, weighting sample, volume, recovery and testing by instrument. Preparing standard solution of the uncertainty of calibration accounted for the largest proportion of the uncertainty, and the uncertainty of volume accounted for the minimum. The expanded uncertainty was 0.476 μg/kg (coverage factor k=2) when the content of OTA was 3.17 μg/kg in cereal grains.Therefore, testing personnel should prepare standard solution strictly in accordance with the specification, at the same time, hold good critical control points of pretreatment process steps such as extraction, purification, elution and so on, in order to control the uncertainty of the determination results in an acceptable range.