Abstract: Some cracks were found on the surface of C12A block plate valve of reheat steam pipe for a ultra supercritical units by magnetic particle testing in a power plant.Through macroscopic examination, chemical composition analysis, metallographic examination, hardness testing and finite element analysis, the cause of cracks was analyzed.The results showed that the stress cracking occurred in block plate valve, the cracks originated from the inclusion defects in block plate valve, and expanded under the mutative temperature and stress.