Abstract: The microstructures and mechanical properties of a modified 310 austenitic stainless steel during isothermal aging at 700℃ were investigated by using scanning electron microscopy(SEM) and transmission electron microscopy(TEM),tensile testing at ambient temperature and 700℃,and micro-hardness testing. It was found that massive block(Cr,Fe,Mo)₂₃C₆,(Cr,Fe)₂₃C₆ carbides,σ-phase and a small quantity of χ-phase formed on grain boundaries and within grains in the tested steel after aging at 700℃ for 1000 h. Precipitates significantly strengthened the mechanical properties of the tested steel at room temperature. The strength increased and the hardness raised by 20 Hv,while the elongation still remained above 30%. However,at high temperature,the effect of precipitation strengthening was weakened,and the ductility slightly decreased. Fracture surface and cross-sectional microstructural analysis after tensile testing showed that the high temperature tensile fracture mode after aging for 1000 h was ductile fracture; neither cracks nor holes were observed; but the room temperature tensile fracture was brittle,and there were some cracks and holes in σ-phase near the fracture. Finally,from the ductile-brittle transition of σ-phase and the difference in strength between room temperature and 700℃,this article discussed the reason for crack and hole forming in the stretching process at room temperature as well as the different effects of aging on the mechanical properties at room temperature and high temperature.