Abstract: In this paper,the composition of the eddy zone is analyzed by means of scanning microscope and the corresponding structure by high magnification metallography.After analyzing a set of photos describing eddy phenomenon,the authors find that the formation of eddy is due to (1) the difference in composition,structure and property of the two welded materials;(2) the characteristic of nugget forming process and (3) the action of volume force such as the electrodynamic force and the medium resistance existing in the liquid nugget.It is pointed out that eddy is essentially a movement of the newly melted metal into the metal melted earlier,and that the rule of nugget growth is best to be observed by the rule of eddy forming.The relation between critical parameters and eddy forming is explained.The properties of joints with eddy are found to be a little lower than those without eddy.