Abstract: From the tidal flat of the Yellow River Estuary,each undisturbed sample of 1 m long was collected before and after cyclic vibrating. The microstructure has been observed with SEM and the transformation characteristics of the granularity have been contrasted at an interval of 10 cm.The results revealed that granularity direction strengthened and the long axes are in a perpendicular trend in general;fine grains gathered in certain areas above 40 cm depth,so the asymmetry coefficient rised and the gradation became better.The granularity change of soil from 40 to 70 cm depth was quite on the contrary;changes below 70 cm depth were not so obvious.The reason may be due to repeat liquefaction of seabed and the variations of seepage field that were induced by cyclic vibration.