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Friction-stir-welded (FSW) Mg alloys usually exhibit an undesirable combination of strength and ductility compared to the base material, posing disadvantages to practical engineering applications. In this work, low-temperature FSW was applied on a non-combustive Mg-9A1-1Zn-1Ca alloy to modify the microstructure and mechanical properties of the welded joint. Results show that many dislocations, second-phase particles, and {10-12} twins were introduced into ultrafine grains of the welded joint, which could randomize the basal texture intensity and enhance strength throughout the joint, leading to fractures in the base material in the transverse tensile test. The appearance of coherent twin boundaries can efficiently accommodate dislocation, thereby elevating ductility and ultimate tensile strength. Keywords: Friction stir welding, Magnesium alloys, Texture, Twin, Microstructure, Mechanical properties