In this study, as-cast WE43 alloy was subjected to submerged friction stir processing (SFSP). Microstructural evolution during SFSP and effect of aging treatment on mechanical properties of SFSP specimens were investigated. The results showed that the microstructure of cast WE43 alloy was significantly refined and the mechanical properties were greatly enhanced. After subsequent artificial aging treatment at 66 h for 150 ℃ and 54 h for 180 ℃, strengths of the peak–aged specimens were further increased, which was attributed to the large quantity of β′′ and β1 precipitates, respectively. Meanwhile, elongations of the peak-aged specimens were both decreased, since more crack nucleation sites were provided with the increased precipitates. The failure mechanism of SFSP specimen was ductile fracture mode, while the peak-aged specimens were all mixed ductile–brittle fracture mode.