Abstract: The chemical behavior of RuNO nitrato complexes in the Purex process is much complicated. Sothe ruthenium decontamination factor(DF_(Ru)) in the uranium purification cycle of a TBP process fallsdown quickly. It is necessary to change the chemical state of RuNO complexes for the ruthenium decon-tamination. Pretreatment with hydrazine nitrate RuNO complexes can be converted into inextractableRu(Ⅲ) and Ru(Ⅳ). However pretreatment coued be done with ascorbic acid instead of hydrazine ni-trate, the results indicate that ascorbic acid pretreatment is effective too. Acidity has more influence onthe result of pretreatment. The lower acidity is, the better the effect will be. Therefore pretreatment isfirst carried out in low acidic solution and then, acidity is raised. In this way, the distribution coeffi-cient of Ru in 30% TBP--kerosene--HNO_3 system can be decreased remarkably.