Abstract: To better understand the stress-corrosion behavior of friction stir welding (FSW), the effects of the microstructure on the stress-corrosion behavior of the FSW in a 2198-T34 aluminum alloy were investigated. The experimental results show that the low-angle grain boundary (LABs) of the stir zone (SZ) of FSW is significantly less than that of heated affected zone (HAZ), thermo-mechanically affected zone (TMAZ), and parent materials (PM), but the grain boundary precipitates (GBPs) T1 (Al₂CuLi) were less, which has a slight effect on the stress corrosion. The dislocation density in SZ was greater than that in other regions. The residual stress in SZ was +67 MPa, which is greater than that in the TMAZ. The residual stress in the HAZ and PM is −8 MPa and −32 MPa, respectively, and both compressive stresses. The corrosion potential in SZ is obviously less than that in other regions. However, micro-cracks were formed in the SZ at low strain rate, which indicates that the grain boundary characters and GBPs have no significant effect on the crack initiation in the stress-corrosion process of the AA2198-T34. Nevertheless, the residual tensile stress has significant effect on the crack initiation during the stress-corrosion process.