The degradation of the strength of a grain boundary was measured by using a micro tensile test in a scanning electron microscope. The change of the crystallinity of grain boundaries during creep-fatigue tests of Ni-base alloy such as Alloy 617 and 625 at elevated temperatures was monitored by electron backscatter diffraction analysis. The image quality (IQ) value obtained from the analysis, which indicates the total density of defects, was applied to the quantitative evaluation of the crystallinity. It was clearly observed that the accumulation of defects occurred at grain boundaries which were perpendicular to the loading direction and consisted of grains with large difference of Schmid factor. Bicrystal specimens with different crystallinity were cut from the tested samples and the strength of the bicrystal specimens were measured by using the micro tensile test system. It was confirmed that the strength of a grain boundary decreased monotonically by about 50% with the decrease of IQ value, in other words, the increase in the total density of various defects such as vacancies and dislocations. On the other hand, the effective yielding stress of grains increased monotonically with the decrease of the IQ value. This is because the increase in the total density of these defects suppresses the movement of dislocations, in other words, plastic deformation. Therefore, there were three independent strengths, the strength of two grains and that of a grain boundary which consisted of the bicrystal specimen. Since the strength of grains increased, at the same time, that of a grain boundary decreased monotonically with the decrease of the IQ value, it was confirmed that there was critical IQ value at which the fracture mode of a bicrystal specimen changed from conventional transgranular cracking to intergranular cracking under the application of uniaxial tensile load.