In contrast to the ordinary ferritic steels, the fracture toughness of 9% Ni steels was found to increase with the strain rate at -196°C. The transition of the fracture mode from brittle to ductile took place at an intermediate strain rate corresponding to the crosshead velocity of 100mm/ min above which the fracture was completely ductile. These observations are attributed to the temperature rise at the crack tip due to the adiabatic heating at high strain rate, which is emphasized by the decreased specific heat and the thermal conductivity at -196°C. The temperature rise during fracture was experimentally confirmed to be 188°C at the maximum.