Abstract: The hot outlet manifold of CO reformer used in a factory cracked prematurely after its service for 2 a. The failure analysis of the cracked outlet manifold was performed by macroscopic observation of the fracture, scanning electron microscope observation, chemical composition analysis and mechanical property tests. The results show that the failure type of the outlet manifold was creep cracking. Because the outlet manifold served at high temperature for a long time, creep cavities generated along grain boundaries and grew gradually at high temperature and stress. The adjacent cavities connected or merged together to form cracks. Cracks propagated gradually and finally the outlet manifold cracked under the stress.