<p>Ni-base superalloys are widely used for various power plants and jet engines. Since the operating temperature of thermal plants and equipment has been increasing to improve their thermal efficiency for decreasing the emission of carbon-dioxide, the initially designed microstructure was found to change gradually during their operation. Since this change of microstructure should deteriorate the strength of the materials, sudden unexpected fracture should occur during the operation of the plants and equipment. Therefore, it is very important to clarify the dominant factor of the change of the microstructure and the relationship between the microstructure and its strength for assuring the stable and reliable operation of equipment. In this study, the change of the strength of a grain of Ni-base superalloys caused by the change of their microstructure was measured by using a micro tensile test system in a focused ion beam system. A creep test was applied to bulk alloys at elevated temperatures and a small test sample was cut from the bulk alloy with different microstructure caused by creep damage by using focused ion beams. The test sample was thinned to 1μm and the sample was stretched to fracture at room temperature. The change of the order of atom arrangement of the sample was evaluated by applying electron back-scatter diffraction (EBSD) analysis quantitatively. In this study, the quality of grains in Ni-base superalloy CM247LC was analyzed by using image quality (IQ) value. It was found that the order of atom arrangement was deteriorated monotonically during the creep tests and this deterioration corresponded to the change of the microstructure clearly. There was a clear relationship between the IQ value of a grain and its strength. Therefore, this IQ value is effective for evaluating the crystallinity of the alloys and the remained strength of the damaged alloys.</p>