Abstract: In the local metallographic examination process for an austenitic stainless steel in-service pressure vessel, the microcracks had been found in the longitudinal welds of its lower head. Formation mechanism of cracks was analyzed, the result showed that thosecracks were composed of transverse liquefaction cracks cause by welding and vertical cracks caused by the suppression moldingin manufacture. Some measures were proposed for the prevention of this kind of cracks.