Abstract: This paper established a process model of arc corrugation on friction-stir weld,which explained the forming reason of arc crest and trough.The softened layer on the interface between shoulder and parentmaterial flew slowly to the back of shoulder in the direction of tool rotation during welding,resulting in softened material accumulating behind shoulder.When the accumulated layer was so thick that the folium outside the layer lagged and formed arc corrugation during cooling,resulting from the inter molecular attraction between shoulder and softened material being in-sufficient to drag all softened material.The above process occurred constantly,resulting in arc trough between two adjacent arc crest.The space between two adjacent are crest depended on tool rotational speed,travel speed and shoulder/parent material interfacial friction coefficient.