Abstract: A method for the synchronous determination of neptunium and plutonium at trace level was developed. Based on the similar chemical behavior at the same oxidation state and physical characteristic of Np and Pu, ²³⁷Np and ²³⁹Pu could be synchronous purified by TOA extraction and Dowex anion exchange double chromatographic columns, and determined by inductively coupled plasma mass spectrometry (ICP MS), using ²⁴²Pu as tracer. The result indicates that Np and Pu have similar chemical behavior in the separation scheme, and less than 1% detection discrepancy of Np and Pu by ICP-MS is obtained. The procedure can be used for different environmental samples after suitable sample preparation.