Abstract The effect of dry grinding on gibbsite was examined by X-ray diffraction, X-ray small angle diffraction and differential thermal analysis. It has been found that there are two sorts of structural change in the process of dry grinding. One, which occurs early in this process, is the slip of layers. In this stage (002) reflection retains its intensity. The other, which takes place after grinding of about 10 hr., is the three-dimensional disintegration of the crystal structure and the intensities of all diffractions become weak. Since only two reflections (002) and (110) are observed after 40 hr., it can be known that the direction of the slipping of layers is [110]. DTA-curve of gibbsite exhibits three endothermic peaks in the beginning of grinding but comes to exhibit only one peak which indicates the decomposition of gibbsite to alumina. This means that ground gibbsite decomposes directly to χ-alumina as fine gibbsite and boehmite is not produced in this process.