Planetary ball mill is used to process kaolinite clay powder under a commercial name of proclay kaolinite. Influence of milling time on structure and thermal behavior of proclay kaolinite are presented and discussed. X-ray powder diffraction shows that peak intensities, at kaolinite characteristic positions, decrease gradually and decay after 3 hours of milling. Amorphous structure of kaolinite forms with a nanocrystalline quartz. The mean crystallite size of nanocrystalline quartz is found to decrease from D = 30 +/- 2 nm for unmilled powder to D = 7 +/- 1.5 nm after 9 hours of processing. For the same time shift, thermal analysis shows that specific heat capacity of unmilled powder increases from 0.93 J/g.K to 1.65 J/g.K for milled powder. (C) 2014 Elsevier B.V. All rights reserved.