The microstructure of gibbsite precipitated with seeds from sodium aluminate solution was studied using SEM. The results show that there are a lot of fine secondary nuclei with diameter less than 1 μm and the surfaces of seeds precipitated by low frequency ultrasound are very irregular. The new nuclei are pseudo-hexagonal slices with nanometer grade in thickness. At higher precipitation temperature, such as 75 ℃, SEM micrographs show that there exist something like colloid on joints of different small granules which have agglomerated to a large particle. It is suggested that the formation of such hexagonal crystalline units is promoted by low frequency ultrasound, while the colloids on the surface may be produced by imcompletely crystallization of the growth units.