The present work studies the AISI304 weld cracking mechanism of AISI321 austenite stainless steel of flue gas pipe,which served up to 60,000 h at 650-700℃. The performance and microstructure of failed material were characterizedby impact test, optical microscopy, scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy(EDS), X-ray diffraction (XRD). The results show that cracks ware caused by embrittlement of weld metal and thermalstress. Needle like r-phase precipitation during service is primarily responsible for significant degradation in mechanicalproperties and microstructures of weld metal. Needle type r-phase is a severe stress-raiser and plays a role as crackinitiator at the interface with matrix. In addition, maximum equivalent stress is about 175 MPa in the elbow whichis near to the yield stress of the steel. It is highly possible that a crack will be initiated under above stress conditionand will propagate faster along embrittled r-phase. Several approaches should be employed to control r-phase precipitationin weld. These approaches include: decreasing content of ferrite and M23C6 carbide in weld and selecting Nbadded weld wire during welding.