Abstract: After 5 years of operation,leakage occurred at the bottom head of the LNG inner tank of a natural gas company. The leakage causes of the tank were analyzed by means of macroscopic inspection, chemical composition analysis, metallographic examination, mechanical property test, fracture microstructure analysis and so on. The results show that deformation induced martensitic transformation occurred in forming process of the head, partial austenite transformed into deformation induced martensite and the microstructure embrittled. During the service time, the straight side of the head subjected to greater internal pressure, combined with the welding residual stress and thermal stress during filling, intergranular crack originated in the head near the welded zone and finally led to fracture failure.