Abstract: The relative value of dissolution or coarse for aging phases in 2024-T351 aluminum alloy during friction stir welding (FSW) process was characterized by differential scanning calorimetry (DSC) method, and a quantitative method to investigate the aging phase behavior during FSW process was revealed. The results show that the dissolution or coarse relative value of aging phases in welded zone of 2024-T351 aluminum alloy could be obtained by formula of f=Sx/SBM-1. Compared with the base metal, the dissolution relative value of S phase in the shoulder active zone (SAZ) of 2024-T351 aluminum alloy was 23.3% when the rotary speed of 700 r·min-1 and welding speed of 200 mm·min-1, and the hardness decreased 23.8%. The dissolution or coarse of S phase depended on the thermal profile generated by FSW.