There is concern that a binary salt of Pu(IV) and Cs forms deposits on the uranyl nitrate hexahydrate (UNH) crystal formed in the dissolver solution for U crystallization containing Cs. Precipitation behavior of dicesium tetravalent plutonium hexanitrate, Cs2Pu(NO3)6, in the U crystallization process is studied. In this work, the solubility of Cs2Pu(NO3)6 was measured in a HNO3 solution, and influence of Pu valence and Cs concentration in the dissolver solution on decontamination factors (DFs) of Pu and Cs in the crystal was examined in the U crystallization process. The solubility of Cs2Pu(NO3)6 increased with a decrease in the concentration of HNO3 in the mother liquor and a rise in temperature of the mother liquor. In the U crystallization process, although the DF of Cs was low where there was Pu(IV) since the two were difficult to separate in the feed solution, Cs was removed thoroughly where there was Pu(VI) in the feed solution. The Cs concentration in the feed solution affected the DFs of Pu and Cs after the UNH crystal was washed. The DFs of Pu and Cs had a tendency to decrease with increase of Cs concentration in the feed solution, because large amounts of Cs+ contributed to the formation of Cs2Pu(NO3)6.