The Al-10wt% Mg alloy was cold-rolled with the thickness reduction ratio of 75% followed by annealing at 75~150 °C, and its microstructure characteristics and mechanical behavior were investigated. The as-rolled and annealed Al-10wt% Mg alloys are characterized by elongated ultra-fine grains, high density of dislocations, a very small amount of Al₃Mg₂ phase, and no dispersed distribution of Al₃Mg₂ phase. With increasing the annealing temperature, the width of elongated ultra-fine grains is increased and the dislocation density is decreased. After annealing at 75~150 °C, compared with those of as-rolled alloys, the yield strength of annealed alloys is decreased by 8%~33%, the ultimate tensile strength is decreased by 1%~12%, and the elongation is increased by 16%~83%. The contributions of various strengthening mechanisms to yield strength and the contributions of preexisting dislocations and Mg solute to ductility were analyzed and discussed.