Abstract: The 9Ni steels at different states, rolling and the ordinary quenching tempering (QT) and interstitial heat treatment(IHT) were studied by crack tip opening displacement (CTOD) test according to BS7448 at -196 ℃. The fracture morphology was observed. The results indicated the specimen after QT treatment showed the best fracture toughness at ultralow temperature, and the fracture toughness of the specimen at rolled state was lower than that after QT treatment and IHT treatment. The fracture morphology of the tabilization expands area of specimens after rolling, QT treatment and IHT treatment were all dimple patterns with the features of ductile fracture.