Abstract: The sealing surface of exhaust valve stem of a diesel engine cracked. The causes of valve stem cracking were analyzed by means of macroscopic observation, chemical composition analysis, mechanical property test, scanning electron microscope and energy spectrum analysis, metallographic examination. The results show that there were hard particles in the service environment of the valve stem. In the process of the valve stem sealing surface sticking to the base, the hard particles caused damage to the sealing surface and formed micro cracks. Under the action of alternating stress, the cracks continued to expand, eventually led to fatigue cracking of the valve stem.