Extruded AZ31Mg alloy was repeatedly alternately compressed in the extrusion direction (ED) and transverse direction (TD) at room temperature. The microstructures of compressed specimens were investigated by various methods, e.g. electron backscattered diffraction, scanning electron microscopy and transmission electron microscopy. The specimen was not fractured even after 10 times alternate compressions, where the strain by each compression was about 12%, although twinning increased with increasing compression number. {
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}-{
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} double twins were generated during ED compression, but barely during TD compression. The double twins differed to those generated during deformation in one direction. {
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} twinning outside the {
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} twin induced {
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} twinning inside the {
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} twin, and consequently, {
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}-{
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} double twins were formed during alternate compression. Hence, the geometric compatibility between {
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} twins inside and outside the {
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} twin was the determinant for the variant selection. Besides, dynamic recrystallization occurred at room temperature in the {
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} twin, which resulted in a high ductility of the alternately compressed specimens.