Abstract: The ratios of ~(146)Pm to ~(147)Pm were determined. When the burnups were 1.5×10~4 and 1.7×10~4 MWd/tU, the ratios were 2.6×10~(-6) and 3.0×10~(-6) respectively.The activity of ~(146)Pm was measured with the γ-ray spectra by a Ge (Li) detectorand that of ~(147)Pm with determination of beta particles by a terphenyl scintillationdetector. The samples of Pm in the spent fuel for a power reactor before measu-rement were separated from Sr, Cs, Ce, U, transuranium elements and others withlevextrel chromatography and then from the other rare earth elements etc. withthe HPLC using α-HIBA.