Abstract: 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 R_239/242m 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.