SCR catalyst installed in a fossil power plant of Taipower had been regenerated due to deactivation after ten years' service. However, a part of the catalyst's substrate was ruptured after two years' reinstall. The destroyed catalyst's substrate was always occurred at the flue gas export comer. The purpose of this article is to find out the reasons of failure for SCR catalyst's substrate by background analysis and site investigation methods which included surface observation, analysis of substrate material, surface composition analysis, microstructure analysis and the profile composition analysis. According to the results of analysis, the main reason of catalyst substrate failure was due to stress corrosion cracking during long-term operation, and the initial region of stress corrosion cracking was occurred at flue gas export corner.