Twining-induced grain refinement in magnesium alloy AZ31 has been studied by using multiple compression,i.e.,changing the loading direction after each pass.The tests were carried out to 50 passes at room temperature with pass strain of 0.05 and 0.1 under a strain rate of 3×10~(-3) s~(-1).The structure evolution is characterized by the formations of c-axis extension twins and c-axis compression twins,and their intersections in various directions,followed by the appearance of fragmented structure. The gradual formation of very fine grains along the c-axis compression twins with increasing cumulative strain finally leads to a ultra-fine grained structure with an average grain size of about 80—150 nm. Low strain may promote the rapid formation of more amounts of mutually crossing twins due to orientation change from pass to pass.The ultra-fine grain evolution can be accelerated by increasing pass strain.The grain refinement mechanisms under cold multiple deformation were discussed in some details.