Abstract: Sources of measurement uncertainty for B(a)P in groundwater samples by High Performance Liquid Chromatography (HPLC) were determined through studying the whole process of analytical results. The sub-uncertainties of the analytical results were evaluated by using a continuous propagation model of uncertainty, and the calibration curve was fitted by using the least squares method. The standard uncertainty for the analytical results of B(a)P in groundwater samples consists of the sub-uncertainties of the sampling volume, the constant volume, determined volume, duplication of the sample, the concentration of the standard solution and the fitting of the calibration curve. Evaluation of the analytical results of two samples with different contents indicate that the lower concentration of B(a)P has the higher relative standard uncertainty. The main sources of measurement uncertainty were the duplication of the sample and the calibration curve fitting.