Abstract: In order to overcome the radioactive contamination and enhance the sensitivity of thermal ionization mass spectrometry, some quantities of uranium of plutonim are electroplated on the single rhenium filament, then a thin layer of platium is overlaied on the filament. The sample atoms thermally diffuse through a platinum layer before they can evaporate as neutral atoms or as ions. Since thermal diffusion is generally a much slower process than direct evaporation and a platinum layer limits the prompt escape of neutral atoms, the filament can be operated at higher temperature, yielding higher ionization efficiency without rapid loss of the sample. The samples of uranium and plutonium are prepared for electrode position by adding 1.0 mol·L -1 HCl to dilute the uranium and plutonium concentrations to 0.1 g/L and 0.01 g/L, respectively. Three kinds of sample plating solutions are the mixtures of the ammonium chloride and chlorhydric acid; the ammonium chloride, chlorhydric acid and oxalic acid; the nitric acid, ammonia and oxalic acid, respectively. Two kinds of platinum solutions in the electrolyte are the chloroplatinic acid, chlorhydric acid and oxalic acid; the platinic sodium hydroxide, sodium hydroxide and oxalic acid, respectively. The ion currents observed for the 100 ng uranium or 50 ng plutonium are 2×10 -14 A during 40~60 min.