The aging treatment for S32205 duplex stainless steel was preformed at 850°C. σ phase precipitation and its effect on mechanical properties of S32205 duplex stainless steel were analyzed by OM, SEM, EDS, hardness measurements and tensile tests. The results show that σ phase begins to nucleate at γ/α or α/α interfaces after 15 min aging, and then grows into the surrounding ferrite areas. The ... [Show full abstract] formation mechanism is a transformation of α into γ2 and σ. The amount of σ phase increases with prolonging aging time, which achieves 18% when aging for 240 min, and the formed σ phase in S32205 duplex stainless steel leads to an obvious increase in the hardness, a certain increase in the strength and a sharp decrease in the ductility. The types of tensile fracture are changing from the typical ductile rupture to the brittle fracture characterized by quasi-cleavage crack.