Abstract: Two different kinds of interphase precipitation of carbides at isotherm al decomposition of 1Cr13 stainless steel, "interphase precipitation" and fibre ca rbide formation, were studied by thin foil TEM, and the relationship between them is also discussed in this paper. The retarding effect of interphase precipitation of carbides on crack propagation was verified by the SEM observation on fractography of impact specimens and relevant microstructure at declined, surface.Finally,this phenomenon was also confirmed by the observation on primary crack propagation and simultaniously was compared with the specimens after quenching and tempering.