Copper and lead in soil samples were measured by using inductively coupled plasma mass spectrometry(ICP-MS), and the impacts of major source of measurement uncertainty were discussed.Uncertainty components such as sample weighing in the process of digestion, the volume size, preparation of standard solutions, curve fitting, measurement repeatability were analyzed and synthesized according to JJF1059—1999(Evaluation and Expression of Uncertainty in Measurement), and the final expanded uncertainty was given.Such expression of result was more objective and true.