Abstract: The ruthenium decontamination factor in Purex process falls quickly in successive TBP cycles. So,it is necessary to change the chemical states of RuNO complexes in order to improve DFRU in the uranium purification cycle. Hydrazine nitrate is being used to transform RuNO complexes into inextractable Ru(Ⅲ) and Ru( Ⅳ). However, hydrazine nitrate may be inverted into hydrazoic acid which is dangerous and can bring an unstable factor. Pretreat-ment using U (Ⅳ) solution provides another method to improve the decontamination of ruthenium in Purex process.0. 02mol/l U(Ⅳ ) solution can transform RuNO complexes into inextractable species by heating in water bath. The DRu can be decreased by a factor of 10 - 20. U( Ⅳ) pretreatment does not bring any harmful chemical in process.The acidity has a very large influence on the effect of pretreatment. The higher the acidity is,the worse the effect will be.