Abstract: A SUS304 stainless steel tee in an ammonia gas pipeline cracked several times. By using macroscopic observation, penetration testing, chemical composition analysis, hardness testing, mechanical property testing, metallographic examination and so on, the causes for cracks of the tee were analyzed. The results show that:the solid solution treatment of the tee was not enforced normatively, which generated much deformed martensite and dispersed distributed carbide in the microstructure, and the hardness and brittleness of the material were increased; in addition, the internal stress of the tee during forming didn't release completely, as a result of that, the cracks generated on the shoulder of the tee under the maximum stress.