Abstract: Multi-layer and multi-pass filling welding was carried out on austenitic stainless steel plates with thickness of 20 mm with domestic nuclear grade E316L electrode. Then the welds were heat treated at 610℃ for 16 h and accelerated thermal aged at 400℃ for different times (400,2 000,3 500 h), and the evolution of ferrite and precipitated phases in the as-welded and heat-treated weld during thermal aging was studied. The results show that the microstructure of as-welded weld and heat-treated weld both consisted of austenite, ferrite and precipitated phases. With the increase of thermal aging time, the amount of ferrite in the weld increased first and then decreased, and the morphology of ferrite changed from discontinuous mesh to relatively continuous mesh and then to discontinuous mesh. The content of ferrite in the as-welded weld was higher than that in the heat-treated weld. During thermal aging, the precipitated phases in the weld were mainly composed of Cr₂₃C₆ phase and σ phase, and the precipitated phases in heat-treated weld was formed more easily than those in the as-welded weld. With the prolongation of thermal aging time, Cr₂₃C₆ phase and σ phase preferentially precipitated and aggregated at the grain boundaries, and then in ferrite crystals.