[en] An investigation on the decontamination of fission products was made aiming to develop reprocessing technology for recycling the fuels of fast reactor. Relative evaluation index was introduced to respective nuclides and elements of the fission products involved in the spent fuels to evaluate each decontamination coefficient in respects of spacial dose contribution, neutron absorption effects in nuclear reactor, radiological toxicity, heat capacity, impurities, etc. In this study, the spent fuels from the FBR using MOX fuel were used as the subjects. The decontamination coefficient which allows to reduce the radioactivity to 2 mrem/h at a site one meter apart from the source was estimated to be in the order of 10⁸ for Ru, Cs, 10⁷ for Pr, Zr, Eu, Sr 10⁶ for Ag, Ce, Pm and 10³ to 10⁴ for other elements. (M.N.)