Kaolinite powder was ground under atmospheric conditions by a co-axial double rotating cylinders mill [Ultra Fine (UF) mill] and tumbling ball mill to investigate the amorphization behavior of kaolinite. Motions of media balls in both mills were also numerically simulated by the particle element method. The UF mill lead to an amorphous state of kaolinite for a shorter time than the tumbling ball mill. The ball impact energy in the UF mill was about 5 times larger than that of the tumbling ball mill. The simulated total kinetic energy of media balls required for 80% amorphization of the sample by the UF mill was considerably lower than that using the tumbling mill. NMR analysis on Al-27 in the structure of kaolinite revealed that a pentagonal coordination remains in the amorphous state when ground by the UF mill, while it almost disappears in the sample ground by the tumbling mill for a prolonged time.