Abstract: Owing to the extracrdinary low-content of 239Pu in environmental samples, serious interference of the hydride ion. 238UH+ to 239Pu in its ICP-MS determination was still encountered even after radio chemical separation of the matrix; and remarkable effect to the accuracy of analytical results was found by the simple and direct deduction of signal intensity of the interfering spectrum from the signal intensity of the analyte. As shown by the experimental results of the study of physiad measurements of the instruments in vriw to lower the rate of formation of 238UH+, the rate was lowered to more than one order of magnitude when membrane de-solvation installation was used in the nebulizer; the RF power showed only little influence on the formation rate; under the optimum flow rate of nebulizer gas, the formation rate of 238UH+ attained to its minimum. Values of the formation rate of 238UH+ measured by Xs skimmer cone was found to be a little lower than those measured by Xt skimmer cone.