Abstract: The pre-twinned pure magnesium was subjected to equal channel extrusion (ECAP).Optical microscope and electronic universal testing machine were used to study the changes of structure and mechanical properties after 1, 2, and 4 passes.The results show that the 5%pre-compressed sample has finer grains than the original sample under ECAP at 250℃.After two passes of extrusion, the pre-twinned sample has more dynamic recrystallized grains than the uncompressed sample.The grain size becomes more uniform after 4 passes of extrusion.ECAP and pre-twinning can increase the elongation of pure magnesium. Compared with that of the original sample, the elongation of the pre-twinned sample after 4-pass extrusion is increased significantly, which shows that twins play an important role in ECAP deformation of the pure magnesium.