The inner core has a differential rotation relative to the crust and mantle, the relative linear velocity between the solid inner core and the molten outer core is the biggest at the eguator and zero at pole area. As a result, the inner core grows faster at the equator than at the pole area. The gravitational force drives the material flow from the equator to the pole area and makes the inner core remain quasi-orbicular. The corresponding axial symmetric stress field makes c-axes of hexagonal close packed (hep) iron align with inner core's rotation axis, resulting in observed seismic anisotropy. The inner core has a differential rotation relative to the crust and mantle, the relative linear velocity between the solid inner core and the molten outer core is the biggest at the eguator and zero at pole area. As a result, the inner core grows faster at the equator than at the pole area. The gravitational force drives the material flow from the equator to the pole area and makes the inner core remain quasi-orbicular. The corresponding axial symmetric stress field makes c-axes of hexagonal close packed (hep) iron align with inner core's rotation axis, resulting in observed seismic anisotropy.