The changes of amount, distribution and stability of reversed austenite and their effects on the cryogenic toughness of 9Ni steel treated by two--phase region heat treatment were studied with XRD and EBSD. It is found that the cryogenic toughness of the 9Ni steel treated by two--phase region heat treatment is more excellent than that of the one treated by quenching and tempering (QT). When temperature of two--phase region is 650 ℃, the most excellent impact toughness is observed and the impact energy at -196 ℃ reaches to 177 J, at the same time the amount of reversed austenite increases to 10.15%, which indicates that the amount of austenitic has important effect on the cryogenic toughness of 9Ni steel. Results of EBSD show that most of the reversed austenite in the steel treated by QT distribute along the grain boundary, but in the steel treated by two--phase region heat treatment the austenites distribute uniformly along all kinds of boundaries, especially also along the lath martensite boundary in the grain, so the cryogenic toughness can be also improved even the amount of austenitic is lower than that in the steel treated by QT process. The stability analysis of reversed austenite shows that the optimized stability can not be observed under the conditions provided in this work.