The influence of ageing at temperature 700°C for up to 20 000 hours on the deformation, damage and fracture behavior of Alloy 617 has been investigated by two toughness tests. Dense nano γ` phase and carbides are the main precipitates. However, the long-term aged material still shows high toughness. The mechanism has been studied using electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI). The results show that dense nano/micro-twins have been formed in the adjacent of the fracture front during the impact toughness or CTOD testing. Increase of ageing time reduces the number of nano/micro-twins, but they can be observed in all aged materials in spite of different strain rates. This indicates that besides dislocation slip, twinning is another deformation mechanism in long term aged material during the toughness tests. Formation of nano/micro-twins may be one of contributions to high toughness in the aged material, which is termed as twin induced toughening.