The dominant factors of electromigration (EM) resistance of electroplated copper thin film interconnections was investigated from the viewpoint of temperature and diffusion paths in the polycrystalline materials. EM test was performed to the interconnection in order to observe the degradation such as accumulation of copper and voids. Many voids and fracture of the interconnection occurred at the not cathode side but center of the interconnection due to large Joule heating under high current density. In addition, accumulation of copper and voids didn't appear homogeneously along all the grain boundaries. The crystallinity of grain boundaries in the interconnection was evaluated by image quality (IQ) value obtained from electron back-scatter diffraction analysis. The damaged grain boundaries mainly corresponded to grain boundaries with low crystallinity. Therefore, high Joule heating density and high speed diffusion of copper atoms along low crystallinity grain boundaries accelerated EM degradation of interconnections. The change of Joule heating density and activation energy of diffusion paths were evaluated by using annealed interconnections. Annealing of the interconnection improved the crystallinity and this improvement decreased the temperature of interconnection under high current density and increased the activation energy of grain boundary diffusion. These Change results in drastic difference of estimated lifetime of the interconnections. Thus, the lifetime of the interconnections under the EM loading is a strong function of their crystallinity of the interconnections. Therefore it is necessary to evaluate and control the crystallinity of the interconnections quantitatively using IQ value to assure their long-term reliability.