The friction stir welding process (FSW) was developed in the United Kingdomin the early 1990s. During FSW, the frictional heat that is generated is effectively utilized to facilitate material consolidation and eventual joining with the aid of axial pressure. The process is, therefore, a non-fusion welding process. FSW was applied in the current study in order to weld AZ31B-H24 alloy plates. Each of the different zones of the welded joint underwent optical metallographic characterization: the parent material, the Heat Affected Zone (HAZ), the Thermo-Mechanically Affected Zone (TMAZ), and the weld nugget. Optical metallography revealed deformation twinning at the TMAZ, grain refinement at the HAZ and evidence of heavy plastic deformation at the nugget. Creep tests at 100°C, 200°C and 300°C were conducted both on the parent material and on the friction stir welded specimens. Two different creep regimes seem to exist, a high stress regime in which creep is controlled by dislocation climb, and a low stress regime in which Grain-Boundary Sliding (GBS) becomes the dominant mechanism. Transmission electron microscopy of welded and non-welded specimens that underwent creep at 100°C revealed the existence of dislocation segments that do not lie on the basal planes. It is therefore assumed that other slip systems are active, in addition to the  basal slip systems known to be the only ones active in pure magnesium up to about 180°C. The proposed deformation mechanism involves dislocation gliding on basal and non-basal planes assisted by twinning and GBS.