ABSTRACT In this work, FSW was used to join AZ31 and AZ91 magnesium (Mg) alloys to investigate the mechanism behind joint formation and its mechanical performance. The cross-sectional microstructure of the weld zone clearly indicated a strong metallurgical bonding between AZ31 and AZ91 sheets in addition to the mechanical interlocking due to the mixing of both base materials. Uniform hardness (62.9 ± 4.5 HV) was recorded in the weld joint and the values were between AZ31 (59.8 ± 2.4 HV) and AZ91 (78.7 ± 19.5 HV). Tensile test results demonstrated decreased strength (145 ± 4.2 MPa) compared with the base alloys (265 ± 7.3 MPa and 190 ± 4.8 MPa for AZ31 and AZ91, respectively. Ductility was noticed as decreased as reflected by the lower % of elongation for the weld joint compared with both AZ31 and AZ91 base materials. The fractography studies of the samples suggest a brittle mode of fracture in the weld joint similar to that of AZ91 base material.