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Despite magnesium limited formability at room temperature, split sleeve hole cold expansion of AZ31B sheet was successfully implemented. Microstructural analysis showed greater hardness in the immediate vicinity of the notch, which was due to high dislocation density and extensive twinning created because of substantial compressive radial strain during the process. Contribution of twinning in accommodating the tensile tangential strain is believed to be the underlying reason why the sheet endured large plastic strains. Residual stress measurements confirmed variations through the thickness, with the highest value being at the exit side of the sheet. The fatigue performance of cold expanded samples revealed that different degrees of cold expansion result in various levels of life enhancement. Optimum degree of expansion was found to be 5%, which leads to 3 times fatigue life improvement compared to the as-received condition, due to the imposed microstructural changes, including developed compressive residual tangential stresses and the greater hardness at the notch root. Abovementioned factors contribute to enhancement of fatigue life through postponing the crack initiation and impeding the crack propagation. Keywords: Magnesium, Split-sleeve cold expansion, Residual stress, Hardness, Texture evolution