Abstract: The microstructure degradation characteristics of TP304Hstainless steel tubes used in power plantboilers were studied by means of optical microscope,scanning electron microscope,transmission electron microscopeand X-ray diffractometer,and the relationships between theσ-phase content and the life of the tubes were alsogiven.The results show that under constant pressure,the microstructure transformation of TP304H tubescorrelated with operating temperature and time,and as the operating time lasting,chain-like carbides wouldprecipitate along grain boundaries at the design temperature and σ-phase would also precipitate along grainboundaries,especially in triangle ones.Tube bursting must happen if the totalσ-phase content reached 15%,soquantitative method can be used to supervise the life of TP304Htubes by measuring theσ-phase content.