Refining　grains　to　nanoscale　has　been　a　challenge,　especially　for　materials　with　low-melting　points　and　poor　ductility,　such　as　Mg　alloys.　Here　we　report　a　novel　mechanism　of　nanocrystallization　assisted　by　solute　segregation,　by　which　a　nanocrystalline　Mg-Ag　alloy　was　produced　by　moderate　cold　rolling　(similar　to　55%　strain)　followed　by　low-temperature　annealing.　Specifically,　during　cold　rolling　non-basal　＜　c　+　a　＞　dislocations　were　activated　to　form　dislocation　cells　walls,　which　were　pinned　by　Ag　segregation,　and　transformed　into　boundaries　of　nanocrystalline　grains　during　annealing.　(C)　2019　Acta　Materialia　Inc.　Published　by　Elsevier　Ltd.　All　rights　reserved.