The mechanism of the drastic decrease in the lifetime of Ni-base superalloy, Alloy 617 under creep-fatigue loading at elevated temperatures was clarified by using EBSD (Electron Back-Scatter Diffraction) analysis. The degradation process was monitored by using an intermittent creep-fatigue test and EBSD analysis. The change of the crystallinity of grains and grain boundaries was quantitatively analyzed by using the image quality (IQ) value obtained from the EBSD analysis. The IQ value indicated the density of defects such as vacancies, dislocations, local strain, and so on. The decrease in the IQ value corresponded to the decrease in the crystallinity of the observed area. The accumulation of fine voids was found to be accelerated under the creep-fatigue loading, and it caused the drastic decrease of not only the IQ value, but also the strength of the grain boundaries. Intergranular cracking started to occur when the crystallinity of grain boundaries decreased to the critical value due to the degradation of the crystallinity caused by the local accumulation of dislocations and voids around the grain boundaries.