Cast austenite stainless steels (CASS) were widely used in the primary circuit piping of pressurized water nuclear reactors (PWRs), for their excellent mechanical behavior, corrosion resistance and good weldability.CASS, however, were known to have a tendency of thermal aging embrittlement after long term service at mid-temperature (about 280-320℃). The microstructures, micro-mechanical properties and tensile fracture behaviors of Z3CN20-09M stainless steels, thermal aged at 400 ℃ for 2~10⁴ h, were studied in this work. TEM was utilized to observe the microstructure. A nano-indenter was used to investigate the micro-mechanical properties. The tensile tests were completed by an in situ fatigue tester. The tensile surface morphology was observed in SEM and the fracture of thermal aged CASS was examined by an electron probe micro-analyzer (EPMA). The results indicated that the spinodal decomposition and G-phase precipitation distributed in ferrite of the aged CASS. The nano-hardness of ferrite increased and the plastic deformation ability of ferrite phase declined. After long-term aging, the yield strength and ultimate strength increased but the elongation declined. Cleavage features were observed in the ferrite phase of thermal aged CASS. It was also found that micro-cracks initiated at phase boundaries and extended toward the ferrite phase. Ductile dimples and tearing at some areas were shown in the austenite phase in the thermal aged CASS. The spinodal decomposition was considered to be the primary mechanism for the change of the tensile fracture behaviors of Z3CN20-09M stainless steel before and after thermal aging.