Cast austenite stainless steel (CASS) possesses excellent mechanical properties, good workability and high resistance to localized corrosion in chloride environments due to the dual phase microstructure in which the island a-ferrite phase distributes in the g-austenite matrix. So they are widely used in the primary coolant pipes of nuclear power plants. However, undesirable s phase can precipitate in these steels when they are welded or heat treated and it severely decreases the toughness of stainless steels. Although some works have been done to investigate the effect of s phase on mechanical properties of CASS, the mechanism of embrittlement was still lacking. In this work, the effect of s phase on toughness of Z3CN20.09M CASS was investigated, and the embrittlement mechanism of s phase in CASS was discussed by using in situ tensile test, microhardness technology and fracture analysis. It was found that the impact energy of specimens aged at 750 ℃ decreased severely due to the presence of s phases. The (s+g₂) structure formed by the eutectoid decomposition of a phase is very hard and its hardness is much higher than that of austenite. This makes the deformation between (s+g₂) structure and austenite incoordinate in aged specimens. The precipitation of s phase brought more s/g₂ and a/s/g₂ high energy non-coherent boundaries. These boundaries hindered dislocation movements and brought stress concentrations. So cracks initiated at the s/g₂ or a/s/g₂ boundaries preferentially and propagated rapidly when the aged specimen bearded impact stress. The much potential cracking sites (s/g₂ and a/s/g₂ boundaries) in the (s+g₂) structure is the main reason of embrittlement of aged Z3CN20.09M CASS with low toughness.