Abstract: The orthogonal experiment was conducted to optimize the wet ball milling process conditions for ultrafine bentonite powder preparation. The analyses of particle size, Fourier transform infrared spectroscopy (FI-IR), X-ray diffraction (XRD) and thermal properties (DSC-TG) were performed for the prepared ultrafine powder. The results show that the particle sizes of bentonite powder prepared is among 200 to 500 nm and the interlayer spacing is expanded from 2.07nm to 2.25 nm under the best wet ball-milling conditions: milling ball bulk density of 70%, milling time of 7 h, slurry concentration of 20%. The bentonite lattice structure still maintains a good character though slight distortion occurs. The penetration test of ultrafine bentonite grease indicates that the ultrafine powder with smaller particle size and good uniformity has excellent thickening ability.