Abstract: In order to find the cracking causes of head cover of a 304 stainless steel tank used for containing trichlorosilane after using about three years, the failure analysis was performed by optical microscopy, direct-reading spectrometer testing and scan electric microscopy. The results show that the cracking reasons of the tank head cover cracks were the deformation of parts of steel plates in tank head cover, and thus to cause part of austenite changing into deformation martensite and the strength and stress corrosion sensitivity increasing. In addition, the stress corrosion cracking being on heat affected zone was due to the welding residual stress generating and corrosive medium in the positin.