Termwise iteration method, which can be used for uncertainty combination in complicated mathematical models, was described. Compared to general methods, termwise iteration method has significant advantages of simple mathematical operation, good logic and straight comparison of the component uncertainties. As an example, the uncertainty of 239Pu determination by ID-ICP-MS was evaluated using termwise iteration method. The major contributions to the uncertainty budget come from the measurement uncertainty of R239/242m and the uncertainty of 239Pu concentration in 242Pu dilution agent. Therefore, in order to reduce the uncertainty of determination, short-term stability of ID-ICP-MS should be improved and more accurate measurement of 239Pu concentration in 242Pu dilution agent should be performed in real operation procedure.