Friction stir welding (FSW) was applied in the current study in order to butt weld AZ31B-H24 alloy plates. Creep tests were conducted both on the parent material and on the friction stir welded specimens. The microstructure of the AZ31B alloy was found to be unstable under creep conditions. In the case of friction stir welded AZ31B, the material undergoes during FSW both recrystallization and grain growth, then the exposure to temperature during creep yields an extensive additional grain growth. On the other hand, twinning and twin-induced recrystallization occur as well during creep so that ultrafine grains are being created concurrently.