Abstract： In the present paper the moderate temperature (MT) phase of the Ge(111) surface, which was obtained by quenching the Ge(111) sample from about 300 1C to room temperature, has been studied by means of ultra-high-vacuum scanning tunneling microscopy (STM) . It has been found that in addition to diffusion of individual Ge adatoms, the most prominent feature of the MT phase is the concerted motion of Ge adatoms along the walls of the 2×2 domains. The moving adatoms may form either open strings or closed loops. Such concerted motion of Ge adatoms on the Ge(111) surface was predicted previously and has been of much theoretical interests, recently. However, this is the first experimental observation of such motion. Our study shows that the motion is not a result of STM tip interference and that it is always along the <011> directions. On the basis of our STM observations as well as the results of the recent theoretical works some discussions on the characteristics of the concerted motion are given.